MATHCOUNTS School Handbook WARM-UPS

Transcription

1 MATHCOUNTS School Handbook WARM-UPS Answers to the Warm-Ups include one-letter codes, in parentheses, indicating appropriate problem solving strategies, as desribed in the Problem Solving section. It should be noted that the strategies indicated may not be the only applicable strategies. A calculator icon indicates problems which may be more easily solved with a calculator. The following codes will be used in the answer keys: (C) Compute or Simplify (F) Use a Formula (M) Make a Model or Diagram (T) Make a Table, Chart or List (G) Guess, Check and Revise (S) Consider a Simpler Case (E) Eliminate (P) Look for Patterns The answer key to each Warm-Up appears on the following page. A detailed solution to one of the ten problems is also provided on the accompanying answer key, and, as appropriate, a mathematical connection to a problem or an investigation and exploration activity has been noted. MATHCOUNTS Symbols and Notation Standard abbreviations have been used for units of measure. Complete words or symbols are also acceptable. Square units or cube units may be expressed as units 2 or units 3. Typesetting of the MATHCOUNTS handbook and competition materials provided by EducAide Software, Vallejo, California.

2 WARM-UP 1 What is the maximum number of 33/c stamps that can be purchased with 5 dollars? 2. A rep-date occurs when the number formed by the number of the month and the number of the day is the same as the last two digits of the year. For example, 9/8/98 is a rep-date since 98 = 98. In which year of the 21st century will the first rep-date occur? How many different four-digit numbers can be formed using each of the digits in 1999 exactly once? 3. Manu and Janani are playing a coin toss game with a fair penny. Manu gets a point if the penny lands on heads, and Janani gets a point if the penny lands on tails. The score is Janani 9, Manu 7, in a game to 10 points. What is the probability that Janani will win the game? 5. A cube has a surface area of 900 cm 2. What is the number of cubic centimeters in the volume of the cube? Express your answer in simplest radical form What is the degree measure of the indicated angle? What is the units digit of 2 48? The perimeter of a rectangle is 48 units, and its length is twice its width. What is the number of square units in the area of the rectangle? What is the ratio of the number of degrees in the complement of a 60-degree angle to the number of degrees in the supplement of a 60-degree angle? Express your answer as a common fraction All clocks on a NASA space shuttle are set to Mission Elapsed Time (MET). The MET clock is set at midnight and begins when the shuttle is launched. Hence, one hour after liftoff, the shuttle s clock reads 1:00 (MET). If a shuttle launches at 8:09 a.m., at what time that afternoon will the shuttle clock read 4:55 (MET)? 10.

3 ANSWER KEY WARM-UP 1 15 (C) (EP) 3. 4 (FT) 7 8 (TEP) (FM) (M) 7. 6 (SP) (M) (FM) 10. 1:04 (C) SOLUTION Problem #7 FIND OUT What would we like to find? The units digit of the 48th power of 2. CHOOSE A STRATEGY SOLVE IT LOOK BACK A good first attempt would be to simply enter the expression into a scientific calculator. Unfortunately, most calculators only show eight digits in the display, and the value of this expression contains fifteen digits. So that won t work. An alternative is to try smaller powers of 2 to search for a pattern and see where that leads. The pattern of the units digit in powers of 2 is predictable. Notice that 2 1 has units digit 2, 2 2 has units digit 4, 2 3 has units digit 8, 2 4 has units digit 6, 2 5 has units digit 2, and so on. The pattern of units digits is 2, 4, 8, 6, 2, 4, 8, 6,..., and it repeats every fourth power. Since we want the units digit of 2 48, and because 48 is a multiple of 4, the units digit will be the same as it is for 2 4. The answer is 6. Our logic makes sense, so we can have some confidence in our answer. An interesting extension is to look for the units digit of powers of other numbers. The pattern of units digits for powers of any digit 1 9 repeats in a similar way. MAKING CONNECTIONS... to Biology Problem #5 The surface area to volume ratio is of extreme importance in the study of cell size. The average human body contains about 65 trillion cells, so it s not surprising that individual cells are very small. Human red blood cells, for instance, are typically only 7 to 8 microns in diameter. (A micron is one-millionth of a meter.) The diameter of most cells fall within the narrow range of microns. Why are cells so small? One factor is the size of the cell membrane. Cells obtain nutrients and eliminate waste through the cell membrane. As a cell increases in size, its need for nutrients and production of waste increases. Therefore, larger cells require a membrane with a larger surface area for the rapid exchange of materials to the environment. As the size of a cell increases, its surface area to volume ratio decreases, thus making it difficult for a large cell to transport needed nutrients in and harmful wastes out. Evolution has kept cell size relatively small to facilitate these processes. INVESTIGATION & EXPLORATION Problem #7 An obvious pattern emerges when the units digit of the powers of a number are analyzed. Similarly, patterns can be used to solve puzzles. Consider the classic Tower of Hanoi puzzle. Three pegs are on a board. Three disks are on one peg: a large disk is at the bottom, a medium disk is in the middle, and a small disk is on top. The object of the puzzle is to move all three disks to a different peg, while obeying two simple rules: You may only move one disk at a time. You may never place a larger disk on a smaller disk. Try to solve this puzzle. If your solution is as efficient as possible, it should only take you 7 moves. Now, increase the number of disks to 4, then to 5, and more. Record your results of the least number of moves it takes to solve the puzzle depending on the number of disks. Do you see a pattern? Can you explain the pattern?

4 WARM-UP 2 A telephone pole is supported by a steel cable which extends from the top of the pole to a point on the ground 3 meters from its base. When Leah walks 2.5 meters from the base of the pole toward the point where the cable is attached to the ground, her head just touches the cable. Leah is 5 meters tall. How many meters tall is the pole? (Problem submitted by Jane Lataille, P.E.) 2. In linear measure, 7 palms equal 1 cubit, and 28 digits equal 1 cubit. What is the number of cubits in 8 palms, 6 digits? Express your answer as a mixed number. 3. Each new triangle shown below has one more dot per side than the previous triangle. What is the total number of dots on the triangle with 358 dots per side? What percent of the quadrilaterals in the diagram below are parallelograms? 5. Mikela drove 500 miles on her three-wheeler. She rotated a spare tire with the other tires so that all four tires got the same amount of wear. How many miles of wear did each tire accumulate? 6. Start with a positive integer; add 4; multiply by 2; subtract 3; multiply by 2; add 2; divide by 4; subtract 3. If the final result is 6, what was the value of the original integer? Find the least prime number greater than What is the mean of all three-digit numbers that can be created using each of the digits 1, 2 and 3 exactly once? Find the least integer value of x for which 2 x + 7 < What is the positive difference between the greatest and least prime factors of 2000? 10.

5 ANSWER KEY WARM-UP 2 9 (M) (CF) (FSP) 50 (MT) (C) 6. 6 (CMG) (TE) (TP) 9. 4 (CG) (TE) SOLUTION Problem #7 FIND OUT We are asked to find the first prime number greater than CHOOSE A STRATEGY SOLVE IT There are many divisibility rules that can be used. Let s take the odd numbers greater than 2000 in order and check them for divisibility by all primes less than 2500 = 50. (Why?) Also note that every even number is divisible by 2, and not prime, so we don t need to check them. The first number we come to that is prime will be the answer. We know that 2001 isn t prime by using our divisibility rule for 3 (the sum of the digits is 3, so 2001 is divisible by 3). To test if 2003 is prime, use the divisibility rules for some of the smaller primes. It s obviously not divisible by 3 (the sum of its digits is 5), nor by 5 (its units digit is 3), nor by 9 (again, the sum of its digits is 5), nor by 11 (the first and third digit have a sum of 2, while the second and fourth digit have a sum of 3). For larger prime numbers (and even for these ones, if you don t know the divisibility rules), a calculator could be used to check. None of the primes less than 50 evenly divide 2003, so it is prime. LOOK BACK Because 2001 and 2002 are not prime, 2003 is the least prime number greater than MAKING CONNECTIONS... to Measurement Problem #2 The cubit, palm and digit were actually the ancient Egyptians three linear units of measure. The cubit was the length of a man s forearm from the tip of his finger to his elbow. The palm was one-seventh of a cubit, and the digit was one-fourth of a palm. Today, the word digit has different meanings. How do we usually use it in mathematics? How is it being used in this measurement system? Measure your cubit, palm and digit. How do your measurements compare to the Egyptian values? The English system of measurement the system still used in the United States is a bit more standardized than the Egyptian system, but it is similarly confusing to use. When talking about linear distance, the basic unit is the foot. A foot is divided into 12 inches, and inches are continually divided in half to form halves, quarters, eighths, sixteenths, thirty-seconds and even sixty-fourths. Feet are also combined to form yards (3 feet) and miles (5280 feet). Unlike the metric system, which is based on powers of 10, the conversions in the English and Egyptian system evolved through tradition and appear to be somewhat arbitrary. INVESTIGATION & EXPLORATION Problem #6 You can use algebra to show why the following trick works: Choose three different digits. Add 3 to the first digit. Multiply by 10. Add the second digit. Add 3. Multiply by 10. Add the last digit. Subtract 330. The result is a three-digit number consisting of the original digits. Let s say the digits chosen were p, q and r. The process then gives 10(p + 3) = 10p + 30 after the first three steps, 10(10p q + 3) = 100p + 10q after the next three steps, and 100p + 10q r 330 = 100p + 10q + r as the final result. Notice that the result is a three-digit number consisting of the three digits chosen. Create a number trick of your own. Exchange your trick with a partner. Can you tell why your partner s trick works, and can your partner tell why your trick works?

6 WARM-UP 3 Express the reciprocal of 2.3 as a common fraction. 2. For how many positive integers n will 60 n also be an integer? What is the median of the composite integers that are greater than 20 and less than 35? 3. Evaluate ( ) 3. Express your answer as a common fraction. 5. If May 1 falls on a Saturday, what is the sum of all the weekend dates (Saturdays and Sundays) in May? The radius of circle O is 12 inches, and AB and CD are tangent to the circle at B and D, respectively. AB = 16, and CD = 5. What is the sum of the number of inches in OC + OA? The sum of three numbers is 81 and their ratio is 3 : 7 : 17. What is the value of the smallest number? What is the sum of the fifth prime number, the sixth composite number, and the third perfect square? The two arithmetic sequences, 1, 5, 9, 13,... and 1, 6, 11, 16,..., have infinitely many terms in common. What is the sum of the first three common terms? Rectangle ABCD lies in circle D with AB = 6 cm and CE = 4 cm. What is the number of centimeters in the length of diagonal AC? 10.

7 ANSWER KEY WARM-UP (C) (TP) (CT) (C) (MT) (FM) 7. 9 (CG) (CT) (TP) (M) SOLUTION Problem #2 FIND OUT CHOOSE A STRATEGY What do we want to know? The number of integer values that will result when 60 is divided by a positive integer. The value of n must be positive, so we need to check values greater than 0. Using a bit more logic, any value of n greater than 60 will yield a fraction with value less than 1 but greater than 0; hence, we can exclude any value greater than 60. Consequently, let s check all values between 0 and 60. SOLVE IT Employing the strategy identified, and using a calculator, the integers 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30 and 60 all yield an integer value when divided into 60. The answer, then, is 12. LOOK BACK In reviewing the list above, it probably seems obvious that the numbers which yield an integer value also happen to be the factors of 60. That shouldn t be too surprising, because by definition they are one and the same for each integer factor of 60, there is a corresponding integer co-factor (that is, an integer by which the factor can be multiplied to give 60). Therefore, our answer of 12 must be correct. MAKING CONNECTIONS... to the Calendar Problem #5 There s a poem that is supposed to help schoolchildren remember how many days in each month: Thirty days hath September, April, June, and November; All the rest have thirty-one Excepting February alone: Which hath but twenty-eight, in fine, Till leap year gives it twenty-nine. Why is the number of days in February so different? The reason February has 29 days once every four years is fairly easy to explain. The amount of time it takes the Earth to orbit the Sun a year is slightly longer than 365 days; it s actually about days. Adding a day every four years roughly puts the calendar right. But why does February have only 28 instead of 30 or 31 days like the other months? According to a Basque legend, a shepherd in the hills of Euskal Herria was thankful because he had not lost many sheep one season. The shepherd thanked the elements: March Weather, you killed none of my sheep this year, and for that I thank you. But March Weather was proud of his fierce reputation, and he was angry that he might lose that reputation; so, he stole two days from February so that he might have more time to be fierce, and since then, February has had only 28 days. That s just a legend, however. The truth is that February originally had 30 days in leap years, and it had 29 days in other years. August, named after the Roman emperor Augustus, originally had 30 days, too. In an egotistical act by Augustus, he increased the number of days in August to 31, so that it would have as many days as July, the month named for his predecessor, Julius Caesar. The extra day was taken from February. INVESTIGATION & EXPLORATION Problem #8 With a calculator, find the sum of the first seven odd positive integers; that is, find the sum To that, add 15; then, add 17, 19, 21, 23, and so on. What pattern develops? While working with square numbers, you may notice some other interesting facts. For instance, no square number has a units digit of 2, 3, 7 or 8. You may also notice that, for every square number n 2, either n 2 1, n 2 or n is divisible by 3. Can you find any other integers which always divide either n 2 1, n 2 or n 2 + 1, regardless of the integer n?

8 WARM-UP 4 A brick mantel over a fireplace consists of rectangles as shown. What is the total number of rectangles in the pattern? 2. Jared has nine coins in his pocket. They all look alike, but one coin is counterfeit and weighs less than the others. What is the least number of weighings on a balance scale needed to guarantee that the counterfeit coin is found? Compute: (Problem submitted by mathlete Marc Costanzo.) 3. The first term of an arithmetic sequence is 15, and the seventh term is 57. What is the third term of the sequence? 5. The perimeter of a square lot is lined with trees, and there are three yards between the centers of adjacent trees. There are eight trees on a side, and a tree is at each corner. What is the number of yards in the perimeter of the lot? What is the least whole number that is divisible by 7, but leaves a remainder of 1 when divided by any integer 2 through 6? Two small circles with radii 2 cm and 3 cm are externally tangent. A third circle is circumscribed about the first two as shown. What is the ratio of the area of the smallest circle to the area of the shaded region? Express your answer as a common fraction. 8. What is the median of all values defined by the expression 2 x 1, where x is a prime number between 0 and 20? What is the sum of the integer solutions to x + 2 < 5? At Agnesi Middle School, Mr. Eye, Mr. Love and Mr. Problems teach science, mathematics, and history but not necessarily in that order. The history teacher, who was an only child, has the least experience. Mr. Problems, who married Mr. Eye s sister, has more experience than the science teacher. Who teaches science? 10.

9 ANSWER KEY WARM-UP (TP) 2. 2 (MEP) (C) 29 (FTP) (FM) (TP) (FM) (CF) (MG) 10. Mr. Eye (E) SOLUTION Problem #6 FIND OUT We are asked to find the least number that is divisible by 7 and when divided by each of 2 6 leaves a remainder of CHOOSE A STRATEGY SOLVE IT Any number that is 1 greater than a multiple of the LCM of 2 6 will leave a remainder of 1 when divided by 2 6. Consequently, the first step should be to find numbers that are 1 greater than the LCM, and then find the least of those which is divisible by 7. Find the prime factorization of the integers 2 6 to find their least common multiple: = = 2 3 The least common multiple is = 60. Thus, the arithmetic sequence 1, 61, 121, 181,... consists of all the numbers that leave a remainder of 1 when divided by 2 6. The least number in this sequence which is also a multiple of 7 is 30 LOOK BACK When 301 is divided by each of the numbers 2 6, the remainder is 1, and 301 is divisible by 7. By finding the least common multiple of 2 6, we know that we have found the least number for our answer. MAKING CONNECTIONS... to Logic Problem #10 Sir Francis Bacon once said, Men imagine that their minds have the command of language, but it often happens that language bears rule over their minds. And the photographer Minor White said, If we had no words, perhaps we could understand each other better; the burden is ours, however. How words are used is very important to the meaning implied. The study of logical reasoning dissects language and interprets an argument s validity and soundness. For instance, by pulling two premises from the text of this problem, a syllogism a deductive argument that draws a conclusion because a common concept appears in both premises can be formed. Mr. Eye had a sister. The history teacher was an only child. Therefore, Mr. Eye was not the history teacher. Syllogisms, in general, obey the form A B; B C; therefore, A C. In mathematics, such an argument is said to obey the law of transitivity. In the case above, the argument actually takes a slightly different form: A B; C B; therefore, A C. Despite a different appearance, this argument is equally valid. INVESTIGATION & EXPLORATION Problem #1 The brick mantel shown is a 2 10 arrangement of rectangles. Consider a 1 3 arrangement of rectangles. How many total rectangles are there? Consider arrangements of 1 4, 1 5 and 1 6. How many rectangles are there in each of these arrangements? What is the pattern for the number of rectangles that will occur in a 1 n arrangement? How many rectangles are in a 2 3, 2 4, 2 5 or 2 6 arrangement? How many rectangles are in a 3 3, 3 4, 3 5 or 3 6 arrangement? In general, how many rectangles will occur in an m n arrangement of rectangles?

10 WARM-UP 5 The surface area of a cube is 294 square centimeters. What is the ratio of the number of square centimeters in the surface area to the number of cubic centimeters in the volume of the cube? Express your answer as a common fraction. 2. July 4, 1903, was a Thursday. On what day of the week was July 4, 1904? 3. Each of the squares shown is inscribed in a larger square so that the vertices of the inscribed square bisect the sides of the larger square. What fraction of the area of the largest square is shaded? Express your answer as a common fraction. Alia s digital clock read 7:15 a.m. when she left for school. When she returned home 7 hours and 15 minutes later, the clock read 5:55 a.m. because the power had gone off during the day. If her clock automatically reset to 12:00 a.m. when power was restored, at what time that morning did the power return? 5. A car holds exactly six people, but only two of those six people can drive the car. What is the number of ways that the six people can be seated in the car on a drive? 6. Terrell usually lifts two 20-pound weights 12 times. If he uses two 15-pound weights instead, how many times must Terrell lift them in order to lift the same total weight? Find the integer n such that n = The point (4, 3) is reflected over the x-axis and then over the y-axis. What is the sum of the coordinates of the new point? 9. The sides of a regular pentagon are extended to form congruent isosceles triangles as shown. What is m A? Tim and Kurt are playing a game in which players are awarded either 3 points or 7 points for a correct answer. What is the greatest score that cannot be attained? 10.

11 ANSWER KEY WARM-UP (FM) 2. Saturday (P) (MP) 8:35 (C) (FTP) (CM) (CMS) 8. 7 (M) (MP) (EP) SOLUTION Problem #5 FIND OUT CHOOSE A STRATEGY SOLVE IT LOOK BACK We are asked to find the number of ways six people can be arranged in a car, knowing that only two of the six people are able to drive. The Fundamental Counting Principle says to multiply the number of ways each event can happen by the number of ways every other event can happen to determine the total number of arrangements. Determine the number of ways a driver can be chosen, then determine the number of ways others can be placed in the remaining seats. When all of this information is gathered, multiply to find the answer. Only two of the six people are able to drive the car, so there are two ways to choose a driver. The rest is then easy, but remember that each time a seat is filled, there are fewer people to fill the remaining seats. In the front middle seat, there are now five people from whom to choose, because one of the six is the driver. Then, there are two people who have been seated, so there are four people who could fill the front right seat. Similarly, any of three people could fill the rear left, two people could fill the rear middle, and the last person must sit in the right rear seat. Hence, there are = 240 ways for these six people to fill the car. It would be hard to identify all 240 possible arrangements. But consider a simpler example. If there were only three seats, and only one person could drive, there would be = 2 ways to fill the seats. It is fairly easy to see that the method of the Fundamental Counting Principle works in this simpler case, so we can have some certainty that our answer is correct. MAKING CONNECTIONS... to Girolamo Cardano Problem #2 There are mathematicians who can tell on which day of the week a certain date will fall. This old parlor trick, which uses a formula to turn the year, month and date into a day, has fascinated people for many generations. But Girolamo Cardano was no ordinary mathematician he took this trick one step further, and he was able to predict the day on which he would die! Cardano ( ) was an Italian mathematician, physician and astrologer in the sixteenth century. He was the first mathematician to describe negative numbers and to comprehend the existence of negative roots. He was also the first to recognize imaginary numbers. Cardano advanced the study of algebra and pioneered the study of probability. Although completely brilliant, Cardano was also a compulsive hypochondriac, continually complaining that he could die at any minute. Finally, he became positive that he would die on September 20, 1576, four days before his 75th birthday. However, nothing happened during the day of September 20, so to prove his prediction correct, he drank a glass of poison that evening. INVESTIGATION & EXPLORATION Problem #10 The answer to problem #10 can be found with a fairly simple formula: if the two values possible in a game are p and q, the greatest impossible score is pq p q. In this case, the greatest score that cannot be attained is 3(7) 3 7 = 11 points. From 0 through 11 points, what scores cannot be attained? What interesting pattern emerges? Try values other than 3 and 7 to see if a similar pattern of unattainable scores results.

12 WARM-UP 6 Angel wants to sell 50 identical pencils in groups of 2 or 3. In how many ways can the pencils be grouped? 2. Eight cubes form the figure shown. If the side length of each cube is 3 cm, how many square centimeters are in the surface area of the figure? Evaluate 3 (7!)(7!)(8!). 3. For what value of x does 3 2x2 5x+2 = 3 2x2 +7x 4? Express your answer as a common fraction. 5. The slant height of a cone is 13 cm, and the height from the vertex to the center of the base is 12 cm. What is the number of cubic centimeters in the volume of the cone? Express your answer in terms of π Given five segments of length 2, 3, 5, 8 and 13, what is the number of distinct triangles that can be formed using any three of the segments? How many squares of any size are in this figure? Two numbers are chosen at random, with replacement, from the set {1, 2, 3, 4}. The two numbers are used as the numerator and denominator of a fraction. What is the probability that the fraction represents a whole number? Express your answer as a common fraction What is the least possible positive integer with exactly five distinct positive factors? A digital, 12-hour clock shows hours and minutes. During what fraction of the day will the clock show the digit 1 in its display? Express your answer as a common fraction. (Problem submitted by alumnus Michael Iachini.) 10.

13 ANSWER KEY WARM-UP 6 9 (TP) (CM) 3. 10,080 (CP) 1 2 (M) π (FM) 6. 0 (TEP) (TP) (TP) (EP) (TP) SOLUTION Problem #9 FIND OUT CHOOSE A STRATEGY SOLVE IT LOOK BACK What do we want to know? The least positive integer with five distinct positive factors. The number of factors an integer has can be found by looking at the prime factorization. If a number factors to 2 a 3 b 5 c, the number of factors can be found with the formula (a + 1)(b + 1)(c + 1). For instance, the number 12 is prime factored as , so it has (2 + 1)(1 + 1) = 6 factors. Let s use this knowledge to find the number for which we are looking. In this problem, the number to be found must have 5 factors. Hence, the number must have the form p 4, for some prime number p. Because the smallest possible positive integer for the answer is required, choose the smallest prime for p. The smallest prime is 2, so the answer must be 2 4 = 16. Clearly, the integer 16 has 5 factors, namely 1, 2, 4, 8 and 16. Further, each of the integers less than 16 can be checked to see that none of them have exactly 5 factors. Notice, also, that the number to be found must be a perfect square, because only perfect squares have an odd number of factors. (Why?) This could have greatly reduced the search. But since this information wasn t used to find the answer, use it for verification because the answer we identified is a perfect square, we can have confidence in our answer. MAKING CONNECTIONS... to the Fibonacci Sequence Problem #6 The lengths of the segments used in this problem are from the Fibonacci Sequence. Each successive length is the sum of the previous two lengths, and any attempt at building a triangle with three consecutive numbers from this sequence comes up short. Any attempted triangle collapses. Leonardo de Pisa ( ) wrote Liber Abaci, a book that influenced the adoption of Hindu Arabic numerals in Europe. In this book, a theoretical problem about rabbits was introduced, and the problem was based on the Fibonacci sequence. The sequence was not given the name Fibonacci until the 19th century, at which time mathematicians became intrigued with the properties of the sequence and its many connections to probability, the golden ratio, and nature. There are many interesting tricks with the Fibonacci sequence and its relatives. Each is based on mathematics and can be proven algebraically. Try this one! Pick any two numbers (e.g., 5 and 7) to begin a Fibonacci-like sequence; then, generate numbers in the sequence by adding the previous two terms. In this case, the sequence becomes 5, 7, 12, 19, 31, 50, 81, 131,.... Now draw a line between any two numbers. The sum of the numbers before the line will always be the same number as the difference between the second number after the line and the second number in the sequence. For example, if a line is drawn between 50 and 81, the sum of all numbers before the line is 12 The difference between the second number after the line, 131, and the second number in the sequence, 7, is also 12 Cool, huh!?! Try the trick with other numbers. Why does this always work?

14 WARM-UP 7 How many different four-digit numbers can be obtained by using any four of the digits 2, 3, 4, 4 and 4? 2. What is the sum of all values of x for which (x + 3) 2 = 7? Circles A, B and C are tangent as shown. The area of circle A is 16π square centimeters, the area of circle C is 16π square centimeters, and the area of circle B is π square centimeters. What is the number of square units in the area of ABC? 3. For what value of n is the four-digit number 712n, with units digit n, divisible by 18? 5. Some bats were in a cave. Two bats could see out of their right eye, three could see out of their left eye, four could not see out of their left eye, and five could not see out of their right eye. What is the minimum possible number of bats in the cave? (Problem submitted by alumnus Dinesh Patel.) 6. How many different paths are possible in moving from A to B given that you must move down to the right or down to the left? From a bag of coins, 1 3 were given to Mary, 1 5 to Norm, 1 6 to Anna, and 1 4 to Bjorn. The six left were given to Troy. How many coins were originally in the bag? Evaluate: (2 + 3) 1 ( ) The chickens and pigs in Farmer McCoy s barn have a total of 50 heads and 170 legs. How many pigs are in the barn? 10. A slug climbs ten inches in ten minutes. It then rests two minutes. It continues climbing at a constant rate and rests for two minutes after climbing ten minutes. How many minutes will it take the slug to reach the top of a twenty-foot tower? (Problem submitted by mathlete Lance Worth.)

15 ANSWER KEY WARM-UP 7 20 (TP) 2. 6 (F) (FM) 8 (EP) 5. 7 (TE) (TP) (CG) (C) (MG) (P) SOLUTION Problem #3 FIND OUT CHOOSE A STRATEGY SOLVE IT What are we asked to find? The area of ABC. To determine the area of a triangle, the base and height of the triangle must be found. The information provided about the areas of the circles can be used to determine the radius of each circle, and adding the radii will determine the lengths of the sides of the triangle. The Pythagorean theorem can then be used to calculate the height of the triangle, and from that the area can be calculated. Circle A has area 16π, and the formula for the area of a circle is πr 2. Hence, the radius of circle A is 4 cm. Likewise, the radius of circle C is also 4 cm, and the radius of circle B is 1 cm. ABC is isosceles with congruent sides of length 5 cm and base of length 8 cm. The height from vertex B forms two right triangles with hypotenuse 5 cm and leg 4 cm. The height, then, is = 3 cm. The area of ABC, then, is A = 1 2 bh = 1 2 (8)(3) = 12 cm2. LOOK BACK Does the answer make sense? Yes. The area of circle A is 16π cm 2, or approximately 50 cm 2. By visual comparison, it seems reasonable that the area of ABC is roughly one-fourth the area of circle A. MAKING CONNECTIONS... to Pythagorean Theorem Problem #3 Although the theorem about the lengths of the sides of right triangles was named the Pythagorean theorem because it was associated with the Pythagorean school, variations on the proof of the theorem have been found throughout the centuries, in different cultures and on various continents. Even United States President James Garfield developed a proof based on two ways of determining the area of a trapezoid. In the figure shown, the area of the trapezoid can be found in two different ways, and these expressions can be set equal. The first way uses the typical formula, which multiplies the average of the bases by the height. The second method finds the area by adding the areas of the three right triangles which comprise the trapezoid. 1 ab (a + b)(a + b) = ab 2 + cc 2 a 2 + 2ab + b 2 = ab + ab + c 2 a 2 + b 2 = c 2 INVESTIGATION & EXPLORATION Problem #4 All prime numbers greater than or equal to 7 share a divisibility rule. We can illustrate the rule by testing 68,198 for divisibility by 13. Starting with 0, list the first ten multiples of 13: 0, 13, 26, 39, 52, 65, 78, 91, 104, 117. Then add to or subtract from 68,198 the multiple of 13 that will result in a 0 as the units digit. In this case, add 52 to give 68, = 68,250. Truncate the units digit from the answer, which gives Then, repeat this process until you get either 0 or a number from 1 to 12. Continuing this example, = = = 0. Because the final result is 0, the original number is divisible by 13; however, had the result been a number from 1 to 12, the original number would not be divisible by 13. This technique may be applied to testing for divisibility for any prime number greater than or equal to 7. Create a few multiples of 7, or 17, or 23, and investigate this technique. Can you explain why it works?

16 WARM-UP 8 For what value of n is the five-digit number 7n,933 divisible by 33? 2. Ben performed the following incorrect operations on a number. First he added 5 instead of subtracting 5. Then he multiplied his result by 4 1 instead of dividing by 4 1. Finally, he squared the last result instead of taking the square root. Ben s final result was If Ben had performed the correct operations, what would the result have been? 3. If each of the variables represents a different digit, what is the value of a + b + c + d? abc + dca 1000 A four-digit number is created by using each of the digits 4, 5, 8 and 9 exactly once. What is the probability that the number will be a multiple of 4? Express your answer as a common fraction. 5. Each fair spinner below is divided into four congruent regions. Joe used spinner A, and Sally used spinner B. They added the results. What is the probability that the sum was even? Express your answer as a common fraction Mrs. Read can knit one pair of children s mittens with a ball of yarn six inches in diameter. How many pairs of identical mittens can she knit with a ball of yarn twelve inches in diameter? Assume that the balls of yarn are rolled consistently. 7. Simplify: 5 3! + 5 4! 5 2 5! 8. The complement of an angle is 5 more than four times the angle. What is the number of degrees in the measure of the angle? 9. What is the total number of square units in the shaded regions of the 3 4 grid of unit squares? Express your answer as a common fraction What is the value of the following expression? Express your answer as a common fraction. 11( ) 10.

17 ANSWER KEY WARM-UP 8 5 (EP) (FS) (EP) 1 6 (TP) (TP) 6. 8 (FM) 7. 5 (C) (FM) (M) (CP) SOLUTION Problem #3 FIND OUT CHOOSE A STRATEGY SOLVE IT What values do we need to identify? The values indicated by the variables a, b, c and d in the addition shown. Relying on logic will solve this problem. Begin by realizing that the sum of the units digits must have a sum of 0. Hence, a + c must equal 10. From there, other values can be tested. The first possibility is to let c = 1 and a = 9. But we may notice immediately that this presents a problem. The addition then becomes 9b1 + d and that can only have a solution if b = 8 and d = 0. However, to have a hundreds digit of 0 makes no sense. So, try a different combination. By letting c = 2 and a = 8, the numbers fall into place to give = The sum of the digits is a + b + c + d = = 18. LOOK BACK There are several possible combinations of digits that will work in this problem. However, the sum of the digits is 18 in each case. For more fun with such problems, check out any of the Alphametic Puzzle Solvers on the Internet. For instance, the site tcollins/alphamet/alpha solve.html will let you enter two addend words and a sum word and then return all possible numeric solutions. MAKING CONNECTIONS... to Knitting Problem #6 Hazel Read has been known as the mitten lady in Littleton, MA, for over 40 years. Everyone looks forward to her hand-knitted mittens at the town Holiday Bazaar each year, and it is a sure thing that all the mittens will sell. She has, indeed, become a legend in her own time. A rumor once circulated that each year she knits as many pairs of mittens as there are years in her age, and that she had done so from age 70 until she turned 90. Hazel says she can t take credit for that many mittens, but at 96, she continues to keep the hands of many Littleton residents warm. But suppose Hazel Read really did live up to the legend? If a ball of yarn 6 inches in diameter makes a pair of mittens, what would be the diameter of a single ball of yarn needed to knit all the mittens from age 70 through age 90? A diameter expressed to the nearest foot seems more appropriate than to the nearest inch, considering how much yarn would be needed. When Hazel heard how big the ball of yarn would be, she said, It makes me tired just thinking about it. I ve only done 30 pairs so far this year. INVESTIGATION & EXPLORATION Problem #10 Divide a one-digit number by 9. Divide a two-digit number by 99. Divide a three-digit number by 999. What do you think will happen when a four-digit number is divided by 9999? The pattern that results is fairly obvious. When divided by other numbers, however, the pattern of repeating digits in the decimal representation may not be so clear, though nonetheless interesting. What are the repeating digits when 1 is divided by 7?...when 2 is divided by 7?...when any integer 1 6 is divided by 7? How are the patterns of repeating digits related? How are the patterns different? When an integer is divided by 13, how long is the string of repeating digits? How long is the string when an integer is divided by 7, or by 11, or by 17?

18 WARM-UP 9 Brianna was having a party for 95 guests. Hot dogs are sold in packages of eight; buns are sold in packages of ten. If she purchased the minimum number of packages of each to guarantee at least one hot dog and one bun for each guest, how many more hot dogs than buns did she buy? 2. At 7:40 p.m., Bob passed mile marker 13 At 8:20 p.m., he passed mile marker 176. What is the number of miles per hour in his average speed? 3. What percent of the volume of a box can be filled with wooden cubes? Express your answer as a decimal to the nearest tenth. How many quadrilaterals of any size are in the diagram? One number is chosen from the first three prime numbers, and a second number is chosen from the first three positive composite numbers. What is the probability that their sum is greater than or equal to 9? Express your answer as a common fraction. 6. In a sequence, each term is obtained by calculating the sum of the preceding two terms. The eighth term is 81, and the sixth term is 3 What is the fourth term? 7. An advertisement read, Take an additional 10% off any item which is already discounted 30%. A clerk with MATHCOUNTS training said, Those two combined discounts are the same as one n% discount. What is the value of n? 8. The vertices of square EFGH lie on the edges of square ABCD. AE EB = 2 1. What is the ratio of the area of square EFGH to the area of square ABCD? Bertrand s Postulate states that there is at least one prime number between any counting number and its double. How many prime numbers are there between 25 and 50? 10. Two numbers, a and b, are randomly selected without replacement from the set {2, 3, 4, 5, 6}. What is the probability that the fraction a b is less than 1 and can be expressed as a terminating decimal? Express your answer as a common fraction

19 ANSWER KEY WARM-UP 9 4 (CM) (FP) (FM) 36 (TSP) (TP) (TP) (C) (FM) 9. 6 (T) (TEP) SOLUTION Problem #6 FIND OUT What are we asked to find? The value of the fourth term in a sequence whose eighth term is 81, whose six term is 31, and where each term is found by adding the previous two terms. CHOOSE A STRATEGY SOLVE IT We are given the sixth and eighth terms, so it is fairly easy to find the seventh term. We can then work backwards to find the fourth term. Because each term is found by adding the two previous terms, the sum of the sixth and seventh terms equals the eighth term. That is, if we call the seventh term s, then 31 + s = 8 Quite obviously, s = 50. Hence, the sequence has the form...,,, 31, 50, 81,.... The fifth term can be found similarly. It is merely the difference between the seventh and sixth terms, or = 19. Likewise, the fourth term is the difference between the sixth and fifth terms, or = 12. LOOK BACK By beginning with fourth term 12 and fifth term 19, we can reconstruct the sequence:..., 12, 19, 31, 50, 81,.... In this sequence, the sixth term is 31 and the eighth term is 81, which is what the original problem stated. Hence, our answer must be correct. MAKING CONNECTIONS... to Patterns of Prime Numbers Problem #9 For years, mathematicians have searched for a pattern to the prime numbers, yet the sequence of primes appears to be highly irregular. One theorem in number theory states that an approximate value of the nth prime number, for very large values of n, is n( n ). Within prime numbers, however, there are some fairly interesting patterns. One of the naughtiest prime numbers is This number has 258,716 digits, and 25,799 of them are 0 s. Roughly 9.97% of the digits are naught (0). And the prime number has 11,337 digits; of them, 11,336 are the digit 9, representing 99.99% of the digits. How many prime numbers are there? Actually, Euclid proved that there are infinite prime numbers. The reasoning is fairly simple, and it is based on a reductio ad absurdum argument an argument that establishes a contradiction to prove that the premise is false. Assume that there is a greatest prime number, and call it P. Then, compute the product of all prime numbers up to and including P, which is P. To that value, add This result, however, is not divisible by any of the prime numbers up to P, so it must be prime. But the premise stated that P was the largest prime number. This contradiction implies that there must be no largest prime number, so there are infinitely many prime numbers. INVESTIGATION & EXPLORATION Problem #6 Calculate the first twenty or so terms of the given sequence. What is the ratio of the first and second terms of the sequence? What is the ratio of the fifth and sixth terms? Of the nineteenth and twentieth? As the terms of the sequence get increasingly larger, the ratio of consecutive terms approaches a stable value. What is the value of that ratio? Compare the value of that ratio, for large terms in the sequence, with the value How do they compare? What relationship do you think they have? (Hint: What is the solution to the equation x 2 x + 1 = 0?)

20 WARM-UP 10 Two girls and three boys sat in a five-seat row at the movie theater. What is the probability that the two people at each end of the row were both boys or both girls? Express your answer as a common fraction. 2. A quarter is placed on the table. What is the number of quarters that can be placed around the original quarter so that each quarter is tangent to the original quarter and to two other quarters? 3. A square is divided into three congruent rectangles. Then, it is divided diagonally as shown. If the area of the shaded trapezoid is 24 square centimeters, how many centimeters are in the perimeter of the original square? What is the least whole number value of x such that f(x) = x 2 + x + 11 is not prime? 5. If a + b = 8, b + c = 3, and a + c = 5, what is the value of the product abc? 6. Two number cubes, each with the digits 1 6 on the six faces, are rolled. What is the probability that the product of the numbers on the top faces will be greater than 12? Express your answer as a common fraction. 7. In May, the price of a pair of jeans was 250% of its wholesale cost. In June, the price was reduced by 25%. After an additional 50% discount in July, the jeans cost $ What was the number of dollars in the wholesale cost of the jeans? What is the number of square centimeters in the shaded area? What is the remainder when the sum of the first 100 positive integers is divided by 9? 10. A rectangular pool measuring 6 feet by 12 feet is surrounded by a walkway. The width of the walkway is the same on all four sides of the pool. If the total area of the walkway and pool is 520 square feet, what is the number of feet in the width of the walkway?

21 ANSWER KEY WARM-UP (MT) 2. 6 (M) (MP) 10 (TG) (MG) (TE) (C) (FM) 9. 1 (SP) (M) SOLUTION Problem #6 FIND OUT CHOOSE A STRATEGY SOLVE IT What do we wish to know? The probability that the product of the numbers rolled on two number cubes will be greater than 12. It will be easiest to keep track of the possible outcomes with a chart. There are 36 possible outcomes, because each cube has 6 faces, and 6 6 = 36. These outcomes are represented in a 6 6 chart below: There are = 13 products greater than 12. Since there are 36 possible outcomes, the probability is LOOK BACK The possible products range in value from 1 to 36. Because 12 is 3 1 that the probability is close to 1 3. of 36, it seems reasonable MAKING CONNECTIONS... to Engineering Problem #4 If the function y = x 2 10x + 24 were graphed in a coordinate plane, its shape would be a parabola. Examples of parabolas abound. For instance, the shape of a satellite dish, the headlight in an automobile, and even the path traveled by a baseball are all parabolas. One man-made structure that appears to be parabolic is the Gateway Arch of the Jefferson National Expansion Memorial in St. Louis, MO. The history surrounding the Arch dates back to Thomas Jefferson. Jefferson authorized the Louisiana Purchase in 1803, and this acquisition doubled the area of the United States. This marked the beginning of the pioneers exploration of the West. Many settlers used the strategic position of St. Louis as their starting point. That s the reason St. Louis was nicknamed Gateway to the West. The Arch was designed by Eero Saarinen, who won a design competition in At 630 feet, the Arch is the tallest monument in the United States. Although its shape can be approximated by the equation y = ( 2x x), it is not a true parabola. It is actually an inverted catenary curve. (A catenary curve is the shape assumed by a chain when its ends are supported.) The 630-foot span between the legs is equal to the height, and the cross-section of each leg is an equilateral triangle. The design of the Arch allows it to withstand winds up to 150 mph. INVESTIGATION & EXPLORATION Problem #2 Place three quarters on a flat surface so that each is tangent to the other two. What shape is formed when the centers of these quarters are connected? (An equilateral triangle.) Now place a quarter on a flat surface, and then place quarters around it as described in this problem. When the centers of these outer quarters are connected, what shape is formed? (A hexagon.) What relationship is there between the equilateral triangle formed by three quarters and the hexagon formed by seven quarters?

22 WARM-UP 11 Each side of the square shown in Stage 0 measures 1 centimeter. When the pattern is continued, what is the number of centimeters in the perimeter of the figure formed in Stage 50? 2. All sixth-grade students are standing in line from shortest to tallest. Three-fourths of them are less than 5 feet tall; two-thirds are less than feet tall; and twelve are not yet 4 feet tall. There 2. are twice as many between 4 and feet as there are between and 5 feet tall. How many students are standing in line? 3. The units digit of a six-digit number is removed, leaving a five-digit number. The removed units digit is then placed at the far left of the five-digit number, making a new six-digit number. If the new number is 3 1 of the original number, what is the sum of the digits of the original number? 3. How many congruent 4-foot tall cylindrical pipes with an inside diameter of 2 inches are needed to hold the same amount of water as one pipe of the same height with an inside diameter of 12 inches? 5. Evaluate: 6! 4! 2! 0! In the diagram, ABCD is a square. The area of rectangle NFMD is half the area of ABCD, and ND = 1 2 CN. If the area of ABCD is 36 square centimeters, what is the number of centimeters in the perimeter of rectangle ABEM? What is the number of positive factors of 648? Corey is reading a 300-page book. After one hour, he had finished 8% of the book. Assuming that he reads at a constant rate, how many more hours will it take him to read the rest of the book? Express your answer as a decimal to the nearest tenth How many different positive four-digit integers contain each of the digits 0, 1, 2 and 3 exactly once? 10. What is the least value of x for which x = 4 x + 1? 10.