2003 Fermat Problem 15

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?

\textbf{(A)}\ 1\quad \textbf{(B)}\ 10\quad \textbf{(C)}\ 18\quad \textbf{(D)}\ 20\quad \textbf{(E)}\ 25

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Fermat contests here.