2008
AMC 8
Competition
Problems
Solutions
1
2008 AMC 8 Problems
Problem 1
Susan had 50 dollars to spend at the carnival. She spent 12 dollars on food and twice as much on rides.
How many dollars did she have left to spend?
Solution
If Susan spent 12 dollars, then twice that much on rides, then she spent
from
dollars in total. We subtract
to get
Problem 2
The ten-letter code
represents the ten digits
represented by the code word
, in order. What 4-digit number is
?
Solution
We can derive that
,
,
, and
. Therefore, the answer is
Problem 3
If February is a month that contains Friday the
, what day of the week is February 1?
Solution
We can go backwards by days, but we can also backwards by weeks. If we go backwards by weeks, we
see that February 6 is a Friday. If now go backwards by days, February 1 is a
2
Problem 4
In the figure, the outer equilateral triangle has area
, the inner equilateral triangle has area , and the
three trapezoids are congruent. What is the area of one of the trapezoids?
Solution
The area outside the small triangle but inside the large triangle is
between the three trapezoids. Each trapezoid has an area of
. This is equally distributed
.
Problem 5
Barney Schwinn notices that the odometer on his bicycle reads
, a palindrome, because it reads the
same forward and backward. After riding more hours that day and the next, he notices that the
odometer shows another palindrome,
Solution
Barney travels
. What was his average speed in miles per hour?
miles in
hours for an average of
miles per hour.
Problem 6
In the figure, what is the ratio of the area of the gray squares to the area of the white squares?
Solution
Dividing the gray square into four smaller squares, there are gray tiles and
giving a ratio of
white tiles,
.
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Problem 7
If
, what is
?
Solution
Separate into two equations
therefore
and
and solve for the unknowns.
and
,
.
Problem 8
Candy sales from the Boosters Club from January through April are shown. What were the average sales
per month in dollars?
Solution
There are a total of
dollars of sales spread through months, for an average of
.
4
Problem 9
In
a
Tycoon Tammy invested
dollars for two years. During the the first year her investment suffered
loss, but during the second year the remaining investment showed a
gain. Over the two-year
period, what was the change in Tammy's investment?
Solution
After the
loss, Tammy has
dollars. After the
This is an increase in dollars from her original
gain, she has
dollars, a
dollars.
.
Problem 10
The average age of the people in Room A is
. The average age of the people in Room B is
. If the
two groups are combined, what is the average age of all the people?
Solution
The total of all their ages over the number of people is
Problem 11
Each of the
students in the eighth grade at Lincoln Middle School has one dog or one cat or both a dog
and a cat. Twenty students have a dog and
students have a cat. How many students have both a dog
and a cat?
Solution
The union of two sets is equal to the sum of each set minus their intersection. The number of students
that have both a dog and a cat is
.
5
Problem 12
A ball is dropped from a height of meters. On its first bounce it rises to a height of meters. It keeps
falling and bouncing to
a height of
of the height it reached in the previous bounce. On which bounce will it not rise to
meters?
Solution
Each bounce is
, the third
times the height of the previous bounce. The first bounce reaches
, the fourth
required height on bounce
, and the fifth
. Half of
is
meters, the second
, so the ball does not reach the
.
Problem 13
Mr. Harman needs to know the combined weight in pounds of three boxes he wants to mail. However, the
only available scale is not accurate for weights less than
pounds or more than
boxes are weighed in pairs in every possible way. The results are
,
and
pounds. So the
pounds. What is the
combined weight in pounds of the three boxes?
Solution
Each box is weighed twice during this, so the combined weight of the three boxes is half the weight of
these separate measures
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Problem 14
Three
, three
, and three
one of each letter. If
are placed in the nine spaces so that each row and column contain
is placed in the upper left corner, how many arrangements are possible?
Solution
There are ways to place the remaining
remaining
, ways to place the remaining
for a total of
, and way to place the
.
Problem 15
In Theresa's first basketball games, she scored
scored fewer than
and points. In her ninth game, she
points and her points-per-game average for the nine games was an integer. Similarly
in her tenth game, she scored fewer than
points and her points-per-game average for the
games was
also an integer. What is the product of the number of points she scored in the ninth and tenth games?
Solution
The total number of points from the first games is
multiple of by scoring less than
make a multiple of
points, Theresa must score points to have a total of
. To make this a
points. To
, she must score points. The product of these two numbers of points is
.
7
Problem 16
A shape is created by joining seven unit cubes, as shown. What is the ratio of the volume in cubic units to
the surface area in square units?
Solution
The volume is of seven unit cubes which is
cubes which is
. The surface area is the same for each of the protruding
. The ratio of the volume to the surface area is
.
Problem 17
Ms.Osborne asks each student in her class to draw a rectangle with integer side lengths and a perimeter
of
units. All of her students calculate the area of the rectangle they draw. What is the difference
between the largest and smallest possible areas of the rectangles?
Solution
A rectangle's area is maximized when it is shaped like a square, or the two side lengths are closest
together in this case with integer lengths. This occurs with the sides
smallest when the side lengths have the greatest difference, which is
is
. Likewise, the area is
. The difference in area
.
8
Problem 18
Two circles that share the same center have radii
path shown, starting at
Solution
and ending at
meters and
meters. An aardvark runs along the
. How many meters does the aardvark run?
We will deal with this part by part: Part 1: 1/4 circumference of big circle=
Part 2: Big radius minus small radius=
small circle=
6: Same as part 2:
Part 3: 1/4 circumference of
Part 4: Diameter of small circle:
Total:
Part 5: Same as part 3:
Part
E
Problem 19
Eight points are spaced around at intervals of one unit around a
square, as shown. Two of the
points are chosen at random. What is the probability that the two points are one unit apart?
points are chosen at random. What is the probability that the two points are one unit apart?
Solution
The two points are one unit apart at places around the edge of the square. There are
ways to choose two points. The probability is
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Problem 20
The students in Mr. Neatkin's class took a penmanship test. Two-thirds of the boys and
of the girls
passed the test, and an equal number of boys and girls passed the test. What is the minimum possible
number of students in the class?
Solution
Let be the number of boys and be the number of girls.
For and to be integers, must cancel out with the numerator, and the smallest possible value is . This
yields boys. The minimum number of students is
.
Problem 21
Jerry cuts a wedge from a -cm cylinder of bologna as shown by the dashed curve. Which answer choice is
closest to the volume of his wedge in cubic centimeters?
Solution
The slice is cutting the cylinder into two equal wedges with equal area. The cylinder's volume is
. The area of the wedge is half this which is
.
10
Problem 22
For how many positive integer values of
are both
and
three-digit whole numbers?
Solution
If
is a three digit whole number,
must be
must be divisible by 3 and be
. If
is three digits, n
So it must be divisible by three and between 300 and 333. There are
such
numbers, which you can find by direct counting.
Problem 23
In square
square
,
and
. What is the ratio of the area of
to the area of
?
Solution
The area of
is the area of square
subtracted by the the area of the three triangles around
it. Arbitrarily assign the side length of the square to be .
The ratio of the area of
to the area of
is
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Problem 24
Ten tiles numbered through
are turned face down. One tile is turned up at random, and a die is rolled.
What is the probability that the product of the numbers on the tile and the die will be a square?
Solution
The numbers can at most multiply to be
and
. The squares less than
are
. The possible pairs are
and
possibilities giving a probability of
. There are
choices and
.
Problem 25
Margie's winning art design is shown. The smallest circle has radius 2 inches, with
each successive circle's radius increasing by 2 inches. Approximately what percent of the design is black?
Solution
The entire circle's area is
. The area of the black regions is
The percentage of the design that is black is
.
.
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