2010 AMC 10B Exam Solutions
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All of the real AMC 8 and AMC 10 problems in our complete solution collection are used with official permission of the Mathematical Association of America (MAA).
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1.
What is
Solution:
We have that and
As such, their difference is:
Thus, C is the correct answer.
2.
Makarla attended two meetings during her -hour work day. The first meeting took minutes and the second meeting took twice as long. What percent of her work day was spent attending meetings?
Solution:
Note that minutes is hours. The second meeting is then hours. Both meetings take a total of hours then. The percent of Makarla's work day spent attending meetings is
Thus, C is the correct answer.
3.
A drawer contains red, green, blue, and white socks with at least of each color. What is the minimum number of socks that must be pulled from the drawer to guarantee a matching pair?
Solution:
To maximize the number of socks, we want to grab as many single socks as possible before getting a pair.
There are colors, which means that we can draw one sock of each color before drawing a pair.
This means that it takes at least socks to be draw before a pair is guaranteed.
Thus, C is the correct answer.
4.
For a real number define to be the average of and What is
Solution:
We have and
Then
Thus, C is the correct answer.
5.
A month with days has the same number of Mondays and Wednesdays. How many of the seven days of the week could be the first day of this month?
Solution:
Note that days leaves a remainder of when divided by
This means that if the month starts on a Saturday, Sunday, Tuesday, or Wednesday, there will be an uneven number of Mondays and Wednesdays.
Then the month can only start on a Monday, Thursday, or Friday.
Thus, B is the correct answer.
6.
A circle is centered at is a diameter and is a point on the circle with What is the degree measure of
Solution:
Consider the following diagram:
We have that
Since is isosceles, we have that
Since we have that
Thus, B is the correct answer.
7.
A triangle has side lengths and A rectangle has width and area equal to the area of the triangle. What is the perimeter of this rectangle?
Solution:
To find the area of the triangle, we can drop the altitude to the side of length
Then we have a right triangle with one leg and hypotenuse
The other leg has length
The area of the triangle is then
The length of the rectangle is then Its perimeter is
Thus, D is the correct answer.
8.
A ticket to a school play costs dollars, where is a whole number. A group of th graders buys tickets costing a total of \$\(48,\) and a group of \(10\)th graders buys tickets costing a total of \$\(64.\) How many values for \(x\) are possible?
Solution:
Note that must divide both and which means that must divide their greatest common divisor.
The greatest common divisor of and is which means divides
has factors, namely all the powers of up to
Thus, E is the correct answer.
9.
Lucky Larry's teacher asked him to substitute numbers for and in the expression and evaluate the result. Larry ignored the parentheses but added and subtracted correctly and obtained the correct result by coincidence. The number Larry substituted for and were and respectively. What number did Larry substitute for
Solution:
Ignoring the parentheses, Larry would get
Evaluating with the parentheses, one would get
Both of these values are the same, so
Thus, D is the correct answer.
10.
Shelby drives her scooter at a speed of miles per hour if it is not raining, and miles per hour if it is raining. Today she drove in the sun in the morning and in the rain in the evening, for a total of miles in minutes. How many minutes did she drive in the rain?
Solution:
Note that minutes is hours. Let Shelby drive hours in the sun.
Then she drives hours in the rain, which means she travels a total of miles. We have this equals so
Then Shelby drives minutes in the rain.
Thus, C is the correct answer.
11.
A shopper plans to purchase an item that has a listed price greater than $ 100 and can use any one of the three coupons. Coupon A gives off the listed price, Coupon B gives $ 30 off the listed price, and Coupon C gives off the amount by which the listed price exceeds $ 100.
Let and be the smallest and largest prices, respectively, for which Coupon A saves at least as many dollars as Coupon B or C. What is
Solution:
Let be the price of the item. Then coupon A saves Coupon B saves $ 30.
Coupon C will save
We must have that and
This shows that and Therefore
Thus, A is the correct answer.
12.
At the beginning of the school year, of all students in Mr. Well's class answered "Yes" to the question "Do you love math", and answered "No." At the end of the school year, answered "Yes" and answered "No." Altogether, of the students gave a different answer at the beginning and end of the school year. What is the difference between the maximum and the minimum possible values of
Solution:
To minimize we want to have as many kids as possible maintain their answer.
We then need at least percent of the students to change their answer.
To maximize we can have everybody that answered no change their answer, but some who answered yes must stay yes.
We need percent of the people who said yes to stay yes, which means only percent can switch.
This makes the maximum percent of students that can switch percent. The difference is then
Thus, D is the correct answer.
13.
What is the sum of all the solutions of
Solution:
We first take care of the outer absolute value. We have either or
These simplify to
We have cases for each equation. For the first one, we have
Solving both gives us and
For the other equation, we have
Again solving both, we ahve and Note that cannot be negative since that means the original absolute value is negative, which is not possible.
Adding up all the solutions gives us
Thus, C is the correct answer.
14.
The average of the numbers and is What is
Solution:
Recall that the sum of the first integers is
Then, we have that which simplifies to by difference of squares. Dividing gives us
Thus, B is the correct answer.
15.
On a -question multiple choice math contest, students receive points for a correct answer, points for an answer left blank, and point for an incorrect answer. Jesse’s total score on the contest was What is the maximum number of questions that Jesse could have answered correctly?
Solution:
Let be the number of questions Jesse answered correctly and be the number he answered incorrectly.
Then
We have that
Rearranging and simplifying tells us that Since is an integer, its maximum value is
Thus, C is the correct answer.
16.
A square of side length and a circle of radius share the same center. What is the area inside the circle, but outside the square?
Solution:
Drop the altitude from to at
Looking at the side lengths, we see that is a triangle.
This means that the area of sector is
We also have that the area of is
The area of the sector outside the square and inside the circle is then
There are of these regions, which gives us a total area of
Thus, B is the correct answer.
17.
Every high school in the city of Euclid sent a team of students to a math contest. Each participant in the contest received a different score. Andrea's score was the median among all students, and hers was the highest score on her team. Andrea's teammates Beth and Carla placed and respectively. How many schools are in the city?
Solution:
Let there be schools. Then there are participants in the contest.
Since everyone has a unique score, there is also a unique median. This means is odd.
Note there are at least teams, since otherwise a placed wouldn't exist.
We also have at most teams, since otherwise we have Andrea's place as being greater than Beth's.
This means that there are teams.
Thus, B is the correct answer.
18.
Positive integers and are randomly and independently selected with replacement from the set
What is the probability that is divisible by
Solution:
Note that This means that if is divisible by the whole expression is as well.
Since is divisible by we have that is divisible by with probability
Now consider not divisible by For the expression to be divisible by we must have that is divisible by
This means that
The only possibility for this is that one of the factors is mod and the other is mod
For each of the two cases, there is a chance that each of the factors is the desired modulus, for a probability of
There are two cases, which means that this happens with a probability.
The total probability is then
Thus, E is the correct answer.
19.
A circle with center has area Triangle is equilateral, is a chord on the circle, and point is outside What is the side length of
Solution:
Consider the following diagram:
Using the formula for the area of a circle, we have that since it is a radius.
Extend to intersect at Let be the side length of
Then we have that and
We have that is right, which means that we can apply the Pythagorean Theorem. This gives us
Simplifying, we get
Since is positive, we must have that
Thus, B is the correct answer.
20.
Two circles lie outside regular hexagon The first is tangent to and the second is tangent to Both are tangent to lines and What is the ratio of the area of the second circle to that of the first circle?
Solution:
Consider the following diagram:
We can see that the smaller circle is inscribed within an equilateral triangle of side length
The inradius of this equilateral triangle is The area of the circle is then
Let be the center of the larger circle. Drop the perpendicular from to at Draw
We have that is right. Since we also have that is a triangle.
Let Then We also have that is the sum of the height of the hexagon, equilateral triangle, and radius of the circle.
Then
Substituting in we get Simplifying gives us
The area of the larger circle is then
The desired ratio is then
Thus, D is the correct answer.
21.
A palindrome between and is chosen at random. What is the probability that it is divisible by
Solution:
Note that we can express any digit number as This can be expressed in long form as Since in a palindrome, we have that and We can simplify this to get Note that is divisible by This means that must also be divisible by
The only way for this to happen is if is or since is not divisible by
There are options for and options for for a total of palindromes.
The total number of palindromes is since there are options for the thousands digit and options for the hundreds digit.
The desired probability is then
Thus, E is the correct answer.
22.
Seven distinct pieces of candy are to be distributed among three bags. The red bag and the blue bag must each receive at least one piece of candy; the white bag may remain empty. How many arrangements are possible?
Solution:
We can count this with complementary counting. The total number of ways to distribute the candies with no restrictions is
To find the number of invalid arrangements, we have to count the number of ways where either the red or blue bag is empty.
For the case where the red bag is empty, each candy has options for the bag that goes into. There are then arrangements for this case. Similarly, there are arrangements for the case where the blue bag is empty.
There is an overlap of one case where both bags are empty. The final answer is then
Thus, C is the correct answer.
23.
The entries in a array include all the digits from through arranged so that the entries in every row and column are in increasing order. How many such arrays are there?
Solution:
Note that and must be in the top left and bottom right corners respectively. We must also have that and are next to these squares.
We can then case on the center square. Note that the only possible values are or
Case 1: the center is
The is necessarily next to the since there is no other option that is less than
Any number can be in the square next to the but the other two squares are then fixed. There are cases (two places for the two places for the and three choices for the square adjacent to ).
Case 2: the center is
We can case on the position of the If the is in the top right square, the is necessarily next to the
If the is above the then the other two squares are fixed. If it is to the left of the the other two squares can be filled arbitrarily.
Now consider when the is below the There are two spots for the and the square next to the can be any number.
The other two squares are then fixed. This means that this case has a total of We multiply by two since the can be either to the right of or below the
Case 3: the center is
This is similar to case since the is fixed instead of the
The total number of arrangements is then
Thus, D is the correct answer.
24.
A high school basketball game between the Raiders and Wildcats was tied at the end of the first quarter. The number of points scored by the Raiders in each of the four quarters formed an increasing geometric sequence, and the number of points scored by the Wildcats in each of the four quarters formed an increasing arithmetic sequence. At the end of the fourth quarter, the Raiders had won by one point. Neither team scored more than points. What was the total number of points scored by the two teams in the first half?
Solution:
Let be the number of points scored in the first quarter. Let be the common ratio for the Raiders and the difference for the Wildcats.
First, we can establish that is an integer. Assume for the sake of contradiction that it is not and where
For and to all be integers, we must have that and divide We can then let for some integer
Then we have that
Minimizing and we can assume that and This gives us which means that
Testing out this value shows that it does not work. Since this value does not work, no other non-integer rational ratio will work. This means is integral.
Then the sum of the quarter scores for the Wildcats is
We must have that
Trying out values, let Then we have which simplifies to
Looking for the smallest multiple of that leaves a remainder of when divided by we get
With this value, we get and The sum of the scores of the first two quarters for both teams is
Thus, E is the correct answer.
25.
Let and let be a polynomial with integer coefficients such that and What is the smallest possible value of
Solution:
We can define a new function so that has roots at and Then we can factor to get
We can plug in the values of and to get And And And
To minimize we can set Then
Thus, B is the correct answer.