2009 AMC 8 Solutions
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All problems are used with official legal permission of the Mathematical Association of America (MAA).
1.
Bridget bought a bag of apples at the grocery store. She gave half of the apples to Ann. Then she gave Cassie apples, keeping apples for herself. How many apples did Bridget buy?
Small Hint:
Work backward from the apples Bridget kept and gave to Cassie.
Big Hint:
Those apples were the half not given to Ann.
Solution:
We can work backwards, starting with the apples that Bridget kept for herself. Adding the apples that she gave Cassie, she now has apples.
Finally, we multiply this value by since she gave half of her initial apples to Ann. so Bridget started off with apples.
Thus, E is the correct answer.
2.
On average, for every sports cars sold at the local car dealership, sedans are sold. The dealership predicts that it will sell sports cars next month. How many sedans does it expect to sell?
Small Hint:
Use the ratio sports cars to sedans.
Big Hint:
Going from to sports cars multiplies by
Solution:
Set up the proportion Cross-multiplying gives and hence
Thus, D is the correct answer.
3.
The graph shows the constant rate at which Suzanna rides her bike. If she rides a total of half an hour at the same speed, how many miles will she have ridden?
Small Hint:
Read one clear point from the graph.
Big Hint:
If minutes gives miles, then minutes is twice as long.
Solution:
From the graph, we can see that Suzanna rides miles in minutes. This means that in minutes, she will have ridden miles.
Thus, C is the correct answer.
4.
The five pieces shown below can be arranged to form four of the five figures below. Which figure cannot be formed?
Small Hint:
Look for a feature in the five pieces that must appear in the final figure.
Big Hint:
The long -square piece must fit as an unbroken segment somewhere.
Solution:
Note that option B does not have any segments in it that are blocks long. This means that it is impossible to arrange the block long piece to fit within the figure.
Thus, B is the correct answer.
5.
A sequence of numbers starts with and The fourth number of the sequence is the sum of the previous three numbers in the sequence: In the same way, every number after the fourth is the sum of the previous three numbers. What is the eighth number in the sequence?
Small Hint:
Generate the sequence one term at a time.
Big Hint:
Each new term is the sum of the previous three terms.
Solution:
The sequence begins
The next terms are and
Thus, D is the correct answer.
6.
Steve’s empty swimming pool will hold gallons of water when full. It will be filled by hoses, each of which supplies gallons of water per minute. How many hours will it take to fill Steve’s pool?
Small Hint:
First find the combined filling rate of the four hoses.
Big Hint:
Convert minutes to hours after finding the total number of minutes.
Solution:
The hoses together fill the pool with gallons of water per minute.
To fill gallons, it will take the hoses minutes to fill the pool.
minutes is the same as hours.
Thus, A is the correct answer.
7.
The triangular plot of land lies between Aspen Road, Brown Road and a railroad. Main Street runs east and west, and the railroad runs north and south. The numbers in the diagram indicate distances in miles. The width of the railroad track can be ignored. How many square miles are in the plot of land
Small Hint:
Use as a base for triangle
Big Hint:
The perpendicular distance from to the railroad is shown by
Solution:
The base of is which is The altitude is as well.
Therefore, the area of is
Thus, C is the correct answer.
8.
The length of a rectangle is increased by and the width is decreased by What percent of the old area is the new area?
Small Hint:
Multiply the scale factors for length and width.
Big Hint:
The new area is times the old area.
Solution:
Let the old length and width be and so the old area is
The new length is and the new width is Thus the new area is
This shows that the new area is of the old area.
Thus, B is the correct answer.
9.
Construct a square on one side of an equilateral triangle. On one non-adjacent side of the square, construct a regular pentagon, as shown. On a non-adjacent side of the pentagon, construct a regular hexagon. Continue to construct regular polygons in the same way, until you construct an octagon. How many sides does the resulting polygon have?
Small Hint:
Count only the outside sides of each polygon.
Big Hint:
Middle polygons share two sides; the two end polygons share one side each.
Solution:
The triangle and octagon are at the ends of the chain, so each loses one side to a shared edge. The square, pentagon, hexagon, and heptagon are in the middle, so each loses two sides to shared edges.
The resulting polygon has sides.
Thus, B is the correct answer.
10.
On a checkerboard composed of unit squares, what is the probability that a randomly chosen unit square does not touch the outer edge of the board?
Small Hint:
Only the squares strictly inside the border do not touch the edge.
Big Hint:
An board has a interior.
Solution:
There are squares on the interior.
This means that the probability of choosing one of these squares is
Thus, D is the correct answer.
11.
The Amaco Middle School bookstore sells pencils costing a whole number of cents. Some seventh graders each bought a pencil, paying a total of dollars. Some of the sixth graders each bought a pencil, and they paid a total of dollars. How many more sixth graders than seventh graders bought a pencil?
Small Hint:
The pencil price must divide both total costs in cents.
Big Hint:
Use the fact that at most sixth graders bought pencils to rule out a -cent price.
Solution:
The number of seventh graders that bought a pencil is divided by the price of a pencil. Similarly, the number of sixth graders that bought a pencil is divided by the price of a pencil.
This means that the price of a pencil divides both and Prime factorizing, we get and The only numbers that divide both and are and
If cent was the price of the pencil, that means sixth graders bought pencils, which is not possible. Therefore, the price of a pencil is cents.
This means that seventh graders bought a pencil, and sixth graders bought a pencil. Therefore, more sixth graders than seventh graders bought pencils.
Thus, D is the correct answer.
12.
The two spinners shown are spun once and each lands on one of the numbered sectors. What is the probability that the sum of the numbers in the two sectors is prime?
Small Hint:
Make a table of possible sums.
Big Hint:
Only sums of are not prime among the possible outcomes.
Solution:
We can find the sum of the two numbers in every possible outcome.
There are only outcomes where the sum is not prime (the two instances when the sum is ). Therefore, the probability that the sum is prime is
Thus, D is the correct answer.
13.
A three-digit integer contains one of each of the digits and What is the probability that the integer is divisible by
Small Hint:
A number is divisible by exactly when its last digit is or
Big Hint:
With digits and each digit is equally likely to be last.
Solution:
The number is equally likely to end in or
It is divisible by only if the last digit is which happens with probability
Thus, B is the correct answer.
14.
Austin and Temple are miles apart along Interstate Bonnie drove from Austin to her daughter’s house in Temple, averaging miles per hour. Leaving the car with her daughter, Bonnie rode a bus back to Austin along the same route and averaged miles per hour on the return trip. What was the average speed for the round trip, in miles per hour?
Small Hint:
Average speed is total distance divided by total time.
Big Hint:
The two -mile trips take different amounts of time.
Solution:
The trip from Austin to Temple took hours. The trip from Temple to Austin took hours. This means that the total time for the round trip was hours.
The total distance of the round trip was miles. Therefore, the average speed for the round trip was miles per hour.
Thus, B is the correct answer.
15.
A recipe that makes servings of hot chocolate requires squares of chocolate, cup sugar, cup water and cups milk. Jordan has squares of chocolate, cups of sugar, lots of water, and cups of milk. If she maintains the same ratio of ingredients, what is the greatest number of servings of hot chocolate she can make?
Small Hint:
Compare how many recipe batches each ingredient can support.
Big Hint:
The ingredient that supports the fewest batches limits the servings.
Solution:
We need to find which ingredient is the limiting factor.
Jordan has enough chocolate for batches, enough sugar for batches, and enough milk for batches.
The milk is limiting, so Jordan can make servings.
Thus, D is the correct answer.
16.
How many -digit positive integers have digits whose product equals
Small Hint:
List the unordered triples of digits whose product is
Big Hint:
Triplets with three distinct digits have permutations, while has
Solution:
The only triples of integers less than that multiply to are
The triples with distinct numbers can be rearranged to form distinct -digit positive integers. The other triple can be arranged to form distinct -digit positive integers.
This leaves a total of integers.
Thus, D is the correct answer.
17.
The positive integers and are the two smallest positive integers for which the product of and is a square and the product of and is a cube. What is the sum of and
Small Hint:
Factor into primes.
Big Hint:
Make exponents even for a square and multiples of for a cube.
Solution:
For a number to be a perfect square, every exponent in the prime factorization must be even. For it to be a cube, the exponents must be divisible by
We can factor to get For to be a perfect square and to be minimized, must have one factor of and one factor of Therefore, we can let
For to be a cube, must have one factor of and two factors of Therefore, we can let suggesting
Thus, B is the correct answer.
18.
The diagram represents a -foot-by--foot floor that is tiled with -square-foot shaded tiles and unshaded tiles. Notice that the corners have unshaded tiles. If a -foot-by--foot floor is to be tiled in the same manner, how many unshaded tiles will be needed?
Small Hint:
Look for the pattern in the unshaded tiles of the floor.
Big Hint:
For an odd side length the unshaded tile count follows
Solution:
In the example, there are rows that contain unshaded tiles, for unshaded tiles.
For a floor with the same pattern, there will be such rows with unshaded tiles each.
Thus the number of unshaded tiles is
Thus, C is the correct answer.
19.
Two angles of an isosceles triangle measure and What is the sum of the three possible values of
Small Hint:
There are three ways the known angle and the angle can sit in an isosceles triangle.
Big Hint:
Consider whether they are the equal angles, or whether one is the vertex angle.
Solution:
All the following possibilities are shown below.
In the first scenario, we get by the properties of the isosceles triangle.
In the second scenario, we get that from which we get that
From the third scenario, we get that from which we get that
The sum of these values yields
Thus, D is the correct answer.
20.
How many non-congruent triangles have vertices at three of the eight points in the array shown below?
Small Hint:
Use symmetry to avoid counting every triangle separately.
Big Hint:
Classify triangles by their side lengths after choosing vertices from the two-row array.
Solution:
By symmetry, it is enough to list one representative of each possible triangle shape.
One complete list of non-congruent possibilities is and
Every other triangle formed from the eight points is congruent to one of these triangles.
Thus, D is the correct answer.
21.
Andy and Bethany have a rectangular array of numbers with rows and columns. Andy adds the numbers in each row. The average of his sums is Bethany adds the numbers in each column. The average of her sums is What is the value of
Small Hint:
Let be the sum of all entries in the array.
Big Hint:
Andy has average row sum while Bethany has average column sum
Solution:
Let be the sum of all numbers in the array.
Andy’s row sums also add to so their average is Bethany’s column sums also add to so their average is
Therefore
Thus, D is the correct answer.
22.
How many whole numbers between and do not contain the digit
Small Hint:
Count by number of digits, or pad numbers with leading zeroes.
Big Hint:
For three padded digits, each position has choices if digit is forbidden, but exclude
Solution:
We can case on the number of digits.
There are one digit numbers excluding
There are two digit numbers that lack the digit
There are three digit numbers that do not include
This yields a total of numbers that do not contain the digit
Thus, D is the correct answer.
23.
On the last day of school, Mrs. Wonderful gave jelly beans to her class. She gave each boy as many jelly beans as there were boys in the class. She gave each girl as many jelly beans as there were girls in the class. She brought jelly beans, and when she finished, she had six jelly beans left. There were two more boys than girls in her class. How many students were in her class?
Small Hint:
Let the number of girls be so the number of boys is
Big Hint:
The total jelly beans given out is
Solution:
Let be the number of boys in the class and be the number of girls. From the problem, we get that
If each boy gets jelly beans, then Mrs. Wonderful will give out a total of jelly beans to all the boys. Similarly, she will give out jelly beans to all the girls.
Therefore, Since cannot be negative, we get that This means that so
Thus, B is the correct answer.
24.
The letters and all represent different digits. If and what digit does represent?
Small Hint:
Use the ones column of the addition first.
Big Hint:
The subtraction forces a borrowing relation involving
Solution:
From the ones column of the addition, ends in so
In the subtraction we now have The ones column requires a borrow, so giving
Then so In the addition, so
Thus, E is the correct answer.
25.
A one-cubic-foot cube is cut into four pieces by three cuts parallel to the top face of the cube. The first cut is foot from the top face. The second cut is foot below the first cut, and the third cut is foot below the second cut. From the top to the bottom the pieces are labeled and The pieces are then glued together end to end in the order to make a long solid as shown. What is the total surface area of this solid in square feet?
Small Hint:
Think of surface area by looking from the six directions.
Big Hint:
The four pieces stack back to a unit cube when viewed from either side.
Solution:
Look at the solid from the six coordinate directions.
Viewed from either end, the exposed vertical faces have total height equal to the thickness of piece namely foot. Thus each end view has area square foot.
From each side, the four pieces stack to the side view of the original unit cube, so each side view has area square foot.
From the top and bottom, each view shows four -by- faces, so each has area square feet.
The total surface area is square feet.
Thus, E is the correct answer.