2020 AMC 10A Problems/Problem 9

Problem

A single bench section at a school event can hold either $7$ adults or $11$ children. When $N$ bench sections are connected end to end, an equal number of adults and children seated together will occupy all the bench space. What is the least possible positive integer value of $N?$

$\textbf{(A) } 9 \qquad \textbf{(B) } 18 \qquad \textbf{(C) } 27 \qquad \textbf{(D) } 36 \qquad \textbf{(E) } 77$

Solution

The least common multiple of $7$ and $11$ is $77$ . Therefore, there must be $77$ adults and $77$ children. The total number of benches is $\frac{77}{7}+\frac{77}{11}=11+7=\boxed{\text{(B) }18}$ .

Solution 2

This is similar to Solution 1, with the same basic idea, but we don't need to calculate the LCM. Since both $7$ and $11$ are relatively prime, their LCM must be their product. So the answer would be $7 + 11 = \boxed{\text{(B) } 18}$ . ~Baolan

Video Solution

https://youtu.be/JEjib74EmiY

~IceMatrix

2020 AMC 10A (Problems • Answer Key • Resources)
Preceded by Problem 8	Followed by Problem 10
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2020 AMC 10A Problems/Problem 9

Contents

Problem

Solution

Solution 2

Video Solution

See Also