The side lengths of an equilateral triangle and a square are integers. If the triangle and the square have the same perimeter, which of the following is a possible side length of the triangle?
(A) 1
(B) 10
(C) 18
(D) 20
(E) 25
Suppose that $T$ is the side length of the equilateral triangle and $S$ is the side length of the square. (Both $S$ and $T$ are integers.) Then, since the perimeters of the triangle and the square are equal, we have $3 T=4 S$.

Since $3 T=4 S$ and each side of the equation is an integer, then $T$ must be divisible by 4 because 4 must divide into $3 T$ evenly and it does not divide into 3 .

The only one of the five possibilities which is divisible by 4 is 20 .

(We should check that $T=20$ does indeed yield an integer for $S$, which it does $(S=15)$.)

ANSWER: (D)