2021 AMC 10B Problems

2021 AMC 10B (Answer Key) Printable versions: Wiki • AoPS Resources • PDF
Instructions This is a 25-question, multiple choice test. Each question is followed by answers marked A, B, C, D and E. Only one of these is correct. You will receive 6 points for each correct answer, 2.5 points for each problem left unanswered if the year is before 2006, 1.5 points for each problem left unanswered if the year is after 2006, and 0 points for each incorrect answer. No aids are permitted other than scratch paper, graph paper, ruler, compass, protractor and erasers (and calculators that are accepted for use on the SAT if before 2006. No problems on the test will require the use of a calculator). Figures are not necessarily drawn to scale. You will have 75 minutes working time to complete the test.
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Problem 1

How many integer values of $x$ satisfy $|x| < 3\pi$ ?

$\textbf{(A)} ~9 \qquad\textbf{(B)} ~10 \qquad\textbf{(C)} ~18 \qquad\textbf{(D)} ~19 \qquad\textbf{(E)} ~20$

Problem 2

What is the value if $\sqrt{(3-2\sqrt{3})^2}+\sqrt{(3+2\sqrt{3})^2}$ ?

$\textbf{(A)} ~0 \qquad\textbf{(B)} ~4\sqrt{3}-6 \qquad\textbf{(C)} ~6 \qquad\textbf{(D)} ~4\sqrt{3} \qquad\textbf{(E)} ~4\sqrt{3} + 6$

Problem 3

In an after-school program for juniors and seniors, there is a debate team with an equal number of students from each class on the team. Among the $28$ students in the program, $25\%$ of the juniors and $10\%$ of the seniors are on the debate team. How many juniors are in the program?

$\textbf{(A)} ~5 \qquad\textbf{(B)} ~6 \qquad\textbf{(C)} ~8 \qquad\textbf{(D)} ~11 \qquad\textbf{(E)} ~20$

Problem 4

At a math contest, $57$ students are wearing blue shirts, and another $75$ students are wearing yellow shirts. The $132$ students are assigned into $66$ pairs. In exactly $23$ of these pairs, both students are wearing blue shirts. In how many pairs are both students wearing yellow shirts?

$\textbf{(A)} ~23 \qquad\textbf{(B)} ~32 \qquad\textbf{(C)} ~37 \qquad\textbf{(D)} ~41 \qquad\textbf{(E)} ~64$

Problem 5

The ages of Jonie's four cousins are distinct single-digit positive integers. Two of the cousins' ages multiplied together give $24$ , while the other two multiply to $30$ . What is the sum of the ages of Jonie's four cousins?

$\textbf{(A)} ~21 \qquad\textbf{(B)} ~22 \qquad\textbf{(C)} ~23 \qquad\textbf{(D)} ~24 \qquad\textbf{(E)} ~25$

Problem 6

Ms. Blackwell gives an exam to two classes. The mean of the scores of the students in the morning class is $84$ , and the afternoon class's mean score is $70$ . The ratio of the number of students in the morning class to the number of students in the afternoon class is $\frac{3}{4}$ . What is the mean of the scores of all the students?

$\textbf{(A)} ~74 \qquad\textbf{(B)} ~75 \qquad\textbf{(C)} ~76 \qquad\textbf{(D)} ~77 \qquad\textbf{(E)} ~78$

Problem 7

In a plane, four circles with radii $1,3,5,$ and $7$ are tangent to line $l$ at the same point $A,$ but they may be on either side of $l$ . Region $S$ consists of all the points that lie inside exactly one of the four circles. What is the maximum possible area of region $S$ ?

$\textbf{(A) }24\pi \qquad \textbf{(B) }32\pi \qquad \textbf{(C) }64\pi \qquad \textbf{(D) }65\pi \qquad \textbf{(E) }84\pi$

Problem 8

Mr. Zhou places all the integers from $1$ to $225$ into a $15$ by $15$ grid. He places $1$ in the middle square (eighth row and eighth column) and places other numbers one by one clockwise, as shown in part in the diagram below. What is the sum of the greatest number and the least number that appear in the second row from the top? [asy] /* Made by samrocksnature */ add(grid(7,7)); label(" $\dots$ ", (0.5,0.5)); label(" $\dots$ ", (1.5,0.5)); label(" $\dots$ ", (2.5,0.5)); label(" $\dots$ ", (3.5,0.5)); label(" $\dots$ ", (4.5,0.5)); label(" $\dots$ ", (5.5,0.5)); label(" $\dots$ ", (6.5,0.5)); label(" $\dots$ ", (1.5,0.5)); label(" $\dots$ ", (0.5,1.5)); label(" $\dots$ ", (0.5,2.5)); label(" $\dots$ ", (0.5,3.5)); label(" $\dots$ ", (0.5,4.5)); label(" $\dots$ ", (0.5,5.5)); label(" $\dots$ ", (0.5,6.5)); label(" $\dots$ ", (6.5,0.5)); label(" $\dots$ ", (6.5,1.5)); label(" $\dots$ ", (6.5,2.5)); label(" $\dots$ ", (6.5,3.5)); label(" $\dots$ ", (6.5,4.5)); label(" $\dots$ ", (6.5,5.5)); label(" $\dots$ ", (0.5,6.5)); label(" $\dots$ ", (1.5,6.5)); label(" $\dots$ ", (2.5,6.5)); label(" $\dots$ ", (3.5,6.5)); label(" $\dots$ ", (4.5,6.5)); label(" $\dots$ ", (5.5,6.5)); label(" $\dots$ ", (6.5,6.5)); label(" $17$ ", (1.5,1.5)); label(" $18$ ", (1.5,2.5)); label(" $19$ ", (1.5,3.5)); label(" $20$ ", (1.5,4.5)); label(" $21$ ", (1.5,5.5)); label(" $16$ ", (2.5,1.5)); label(" $5$ ", (2.5,2.5)); label(" $6$ ", (2.5,3.5)); label(" $7$ ", (2.5,4.5)); label(" $22$ ", (2.5,5.5)); label(" $15$ ", (3.5,1.5)); label(" $4$ ", (3.5,2.5)); label(" $1$ ", (3.5,3.5)); label(" $8$ ", (3.5,4.5)); label(" $23$ ", (3.5,5.5)); label(" $14$ ", (4.5,1.5)); label(" $3$ ", (4.5,2.5)); label(" $2$ ", (4.5,3.5)); label(" $9$ ", (4.5,4.5)); label(" $24$ ", (4.5,5.5)); label(" $13$ ", (5.5,1.5)); label(" $12$ ", (5.5,2.5)); label(" $11$ ", (5.5,3.5)); label(" $10$ ", (5.5,4.5)); label(" $25$ ", (5.5,5.5)); [/asy]

$\textbf{(A)} ~367 \qquad\textbf{(B)} ~368 \qquad\textbf{(C)} ~369 \qquad\textbf{(D)} ~379 \qquad\textbf{(E)} ~380$

Problem 9

Problem 10

Problem 11

Problem 12

Problem 13

Problem 14

Problem 15

Problem 16

Problem 17

Problem 18

A fair $6$ -sided die is repeatedly rolled until an odd number appears. What is the probability that every even number appears at least once before the first occurrence of an odd number?

$\textbf{(A)} ~\frac{1}{120} \qquad\textbf{(B)} ~\frac{1}{32} \qquad\textbf{(C)} ~\frac{1}{20} \qquad\textbf{(D)} ~\frac{3}{20} \qquad\textbf{(E)} ~\frac{1}{6}$

Problem 19

Problem 20

Call a positive integer $uphill$ if every digit is strictly greater than the previous digit. For example, 1357, 81, and 5 are all uphill integers, but 32, 1240, and 466 are not. How many uphill integers are divisible by 15?

$\textbf{(A)} ~4 \qquad\textbf{(B)} ~5 \qquad\textbf{(C)} ~6 \qquad\textbf{(D)} ~7 \qquad\textbf{(E)} ~8$

Problem 21

Problem 22

Ang, Ben, and Jasmin each have $5$ blocks, colored red, blue, yellow, white, and green; and there are $5$ empty boxes. Each of the people randomly and independently of the other two people places one of their blocks into each box. The probability that at least one box receives $3$ blocks all of the same color is $\frac{m}{n}$ , where $m$ and $n$ are relatively prime positive integers. What is $m + n ?$

$\textbf{(A)} ~47 \qquad\textbf{(B)} ~94 \qquad\textbf{(C)} ~227 \qquad\textbf{(D)} ~471 \qquad\textbf{(E)} ~542$

2021 AMC 10B (Problems • Answer Key • Resources)
Preceded by 2021 AMC 10A	Followed by 2022 AMC 10A
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