2015 AMC 8 Problems/Problem 20

Ralph went to the store and bought 12 pairs of socks for a total of $24. Some of the socks he bought cost $1 a pair, some of the socks he bought cost $3 a pair, and some of the socks he bought cost $4 a pair. If he bought at least one pair of each type, how many pairs of $1 socks did Ralph buy?

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

Solution

So let there be $x$ pairs of $1$ dollar socks, $y$ pairs of $3$ dollar socks, $z$ pairs of $4$ dollar socks.

We have $x+y+z=12$ , $x+3y+4z=24$ , and $x,y,z \ge 1$ .

Now we subtract to find $2y+3z=12$ , and $y,z \ge 1$ . It follows that $y$ is a multiple of $3$ and $2y$ is a multiple of $6$ , so since $0<2y<12$ , we must have $2y=6$ .

Therefore, $y=3$ , and it follows that $z=2$ . Now $x=12-y-z=12-3-2=\boxed{\textbf{(D)}~7}$ , as desired.

2015 AMC 8 (Problems • Answer Key • Resources)
Preceded by Problem 19	Followed by Problem 21
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All AJHSME/AMC 8 Problems and Solutions

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2015 AMC 8 Problems/Problem 20

Solution

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