1992 AMC 8 Solutions
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All problems are used with official legal permission of the Mathematical Association of America (MAA).
1.
What is the value of the following expression?
Small Hint:
Group the numerator into pairs , and do the same for the denominator
Big Hint:
Each numerator pair equals ; in the denominator group , where each pair is , with a left over
Solution:
Grouping the numerator in pairs gives
Grouping the denominator as gives four pairs of plus , which is
So the expression is
Thus, the correct answer is B .
2.
Which of the following is not equal to
Small Hint:
Rewrite as the mixed number
Big Hint:
A mixed number equals only when ; check each choice
Solution:
Note Now check each choice: and
But
Thus, the correct answer is D .
3.
What is the largest difference that can be formed by subtracting two numbers chosen from the set
Small Hint:
A difference is largest when you subtract the smallest number from the largest number
Big Hint:
Subtracting a negative adds; compute
Solution:
The largest difference uses the largest number, , minus the smallest number,
This gives
Thus, the correct answer is D .
4.
During the softball season, Judy had hits. Among her hits were home run, triple, and doubles. The rest of her hits were singles. What percent of her hits were singles?
Small Hint:
First find how many hits were not singles
Big Hint:
Singles ; divide by and convert to a percent
Solution:
The non-single hits number so the singles number
The fraction of singles is
Thus, the correct answer is E .
5.
A circle of diameter is removed from a rectangle, as shown. Which whole number is closest to the area of the shaded region?
Small Hint:
The rectangle has area ; subtract the area of the circle
Big Hint:
The circle has radius so its area is a little less than
Solution:
The rectangle has area and the removed circle has area
So the shaded region has area whose closest whole number is
Thus, the correct answer is E .
6.
A triangular expression with top entry , lower-left entry , and lower-right entry means , as shown below. What is the indicated sum?
Small Hint:
Apply the rule to each triple separately: top plus lower-left minus lower-right
Big Hint:
Compute and then add the two results
Solution:
The first triple gives and the second gives
Their sum is
Thus, the correct answer is D .
7.
The digit-sum of is How many -digit whole numbers, whose digit-sum is , are even?
Small Hint:
A digit-sum of forces the three digits to be very large
Big Hint:
The only digits that work are and ; list the arrangements and keep the even ones
Solution:
A digit-sum of with three digits requires the digits and The -digit numbers using them are and
Of these, only is even, so there is exactly such number.
Thus, the correct answer is A .
8.
A store owner bought pencils at each. If he sells them for each, how many of them must he sell to make a profit of exactly
Small Hint:
First find the total cost of the pencils
Big Hint:
The revenue must be the cost plus divide that needed revenue by the price
Solution:
The pencils cost To make a profit, the revenue must be
At each, he must sell pencils.
Thus, the correct answer is C .
9.
The population of a small town is The graph indicates the numbers of females and males in the town, but the vertical scale values are omitted. The bars labeled and represent females and males, respectively. How many males live in the town?
Small Hint:
Let the number of males be ; then the number of females is
Big Hint:
Since solve
Solution:
If there are males, then there are females, and together
So giving
Thus, the correct answer is B .
10.
An isosceles right triangle with legs of length is partitioned into congruent triangles as shown. The shaded area is
Small Hint:
The whole triangle has area split into equal pieces
Big Hint:
Each small triangle has area ; count how many are shaded
Solution:
The large triangle has area so each of the congruent small triangles has area
Ten of the small triangles are shaded, so the shaded area is
Thus, the correct answer is B .
11.
The bar graph shows the results of a survey on color preferences. From left to right, the bars represent red, blue, brown, pink, and green. What percent preferred blue?
Small Hint:
Add the five counts to get the total number of people surveyed
Big Hint:
The percent preferring blue is (blue count) divided by (total), converted to a percent
Solution:
The total number surveyed is
The percent preferring blue is
Thus, the correct answer is B .
12.
The five tires of a car (four road tires and a full-sized spare) were rotated so that each tire was used the same number of miles during the first miles the car traveled. For how many miles was each tire used?
Small Hint:
At any moment of the tires are on the ground, so the total tire-mileage is
Big Hint:
Share that total tire-mileage equally among the tires
Solution:
During the miles, tires are always in use, so the total tire-mileage is tire-miles.
Split equally among tires, each tire is used miles.
Thus, the correct answer is C .
13.
Five test scores have a mean (average score) of a median (middle score) of and a mode (most frequent score) of The sum of the two lowest test scores is
not determined by the information given
Small Hint:
The five scores add up to and the middle score is
Big Hint:
The mode must appear at least twice, and both copies must be at or above the median, so the three highest scores are and
Solution:
The five scores sum to The median is the third score, Since is the mode, it must appear at least twice, and both copies lie above the median, so the two highest scores are and
The three highest scores are and summing to So the two lowest sum to
Thus, the correct answer is B .
14.
When four gallons are added to a tank that is one-third full, the tank is then one-half full. The capacity of the tank in gallons is
Small Hint:
Adding gallons raises the tank from full to full
Big Hint:
So gallons equals of the capacity
Solution:
The gallons account for the change from full to full, which is of the capacity.
If of the capacity is gallons, the full capacity is gallons.
Thus, the correct answer is D .
15.
What is the nd letter in this sequence?
ABCDEDCBAABCDEDCBAABCDEDCBAABCDEDC
Small Hint:
The string repeats over and over; count how many letters are in one block
Big Hint:
One block has letters, so divide by and use the remainder to find the position within a block
Solution:
The sequence is the -letter block repeated again and again.
Since the nd letter is the rd letter of a block, which is
Thus, the correct answer is C .
16.
Which cylinder has twice the volume of the cylinder shown at right?
None of the above
Small Hint:
The volume of a cylinder is ; first compute the volume of the given cylinder
Big Hint:
Doubling the height doubles the volume, but doubling the radius multiplies the volume by ; check the and labels on each choice
Solution:
The given cylinder has volume so a cylinder of twice the volume has volume
The choices have volumes (A), (B), (C), and (D). Only (B) equals
Thus, the correct answer is B .
17.
The sides of a triangle have lengths and where is a whole number. What is the smallest possible value of
Small Hint:
For a triangle, the two shorter sides must add to more than the longest side
Big Hint:
You need ; find the smallest whole number
Solution:
By the triangle inequality, must exceed the longest side so
The smallest whole number greater than is and it does form a valid triangle.
Thus, the correct answer is B .
18.
On a trip, a car traveled miles in an hour and a half, then was stopped in traffic for minutes, then traveled miles during the next hours. What was the car’s average speed in miles per hour for the -hour trip?
Small Hint:
Average speed is total distance divided by total time
Big Hint:
Total distance is miles; total time is hours, since the stop still counts
Solution:
The car covered miles in total, over hours (the traffic stop still counts as time).
The average speed is miles per hour.
Thus, the correct answer is A .
19.
The distance between the th and th exits on an interstate highway is miles. If any two exits are at least miles apart, then what is the largest number of miles there can be between two consecutive exits that are between the th and th exits?
Small Hint:
Between the th and th exits there are gaps
Big Hint:
To make one gap as large as possible, make the other gaps as small as allowed, miles each
Solution:
From the th exit to the th exit there are gaps between consecutive exits, each at least miles.
To maximize one gap, make the other gaps exactly miles, using miles. The remaining gap is miles.
Thus, the correct answer is C .
20.
Which pattern of identical squares could not be folded along the lines shown to form a cube?
Small Hint:
A cube has faces, so each pattern has squares; try folding each one up in your mind
Big Hint:
Track where each square lands as the pattern folds; a pattern fails when two squares would cover the same face
Solution:
Each of the five patterns has squares. Folding patterns (A), (B), (C), and (E) wraps the squares neatly onto the six faces of a cube.
For pattern (D), any attempt to fold forces two of the squares to land on the same face, so they overlap and no cube can be formed.
Thus, the correct answer is D .
21.
Northside’s Drum and Bugle Corps raised money for a trip. The drummers and bugle players kept separate sales records. In each pair on the graph, the open bar represents drums and the shaded bar represents bugles. According to the double bar graph of monthly sales below, in what month did one group’s sales exceed the other’s by the greatest percent?
Jan
Feb
Mar
Apr
May
Small Hint:
For each month, read off the two bar heights and find the gap between them
Big Hint:
Divide the difference by the smaller value to find the percent by which the larger exceeds it; compare this ratio across the months
Solution:
Reading the graph gives (drums, bugles): January February March April and May
The percent excess of the larger over the smaller is in January, in February, in March, in April, and in May. The greatest is February, where exceeds by about
Thus, the correct answer is B .
22.
Eight square tiles are arranged as shown so their outside edges form a polygon with a perimeter of units. Two additional tiles of the same size are added to the figure so that at least one side of each added tile is shared with a side of one of the squares in the original figure. Which of the following could be the perimeter of the new figure?
Small Hint:
Adding a tile that shares exactly one side changes the perimeter by ; sharing two sides changes it by
Big Hint:
With two added tiles, the total change can be or ; add these to
Solution:
A tile that shares exactly one side adds to the perimeter (four new edges minus two hidden), while a tile sharing two sides adds
With two added tiles the perimeter can change by or giving new perimeters of or Of the choices, only is possible.
Thus, the correct answer is C .
23.
If two dice are tossed, the probability that the product of the numbers showing on the tops of the dice is greater than is
Small Hint:
There are equally likely outcomes; count those whose product exceeds
Big Hint:
Make a multiplication table and mark every product greater than
Solution:
There are equally likely outcomes. Counting the ordered pairs whose product exceeds : with a first die of there is (namely ); of , there are ; of , there are ; of , there are ; of , there are
That is favorable outcomes, so the probability is
Thus, the correct answer is B .
24.
Four circles of radius are arranged as shown. Their centers are the vertices of a square. The area of the shaded region is closest to
Small Hint:
Since neighboring circles are tangent, the square’s side equals two radii, so its side is and its area is
Big Hint:
The four quarter-circles inside the square together make one full circle of area ; subtract that from the square
Solution:
Because adjacent circles are tangent, the square through the centers has side and area
Inside the square, each circle contributes a quarter-circle, and the four quarters make one full circle of area The shaded region is
Thus, the correct answer is A .
25.
One half of the water is poured out of a full container. Then one third of the remainder is poured out. Continue the process: one fourth of the remainder for the third pouring, one fifth of the remainder for the fourth pouring, and so on. After how many pourings does exactly one tenth of the original water remain?
Small Hint:
After the first pouring remains; after the second remains
Big Hint:
The remaining fraction telescopes: after pourings it is ; set this equal to
Solution:
After the th pouring, the fraction remaining is which telescopes to
Setting gives
Thus, the correct answer is D .