AMC 12 2009b Problem 4

A rectangular yard contains two flower beds in the shape of congruent isosceles right triangles. The remainder of the yard has a trapezoidal shape, as shown. The parallel sides of the trapezoid have lengths $15$ and $25$ meters. What fraction of the yard is occupied by the flower beds?

unitsize(2mm);
defaultpen(linewidth(.8pt));

fill((0,0)--(0,5)--(5,5)--cycle,gray);
fill((25,0)--(25,5)--(20,5)--cycle,gray);
draw((0,0)--(0,5)--(25,5)--(25,0)--cycle);
draw((0,0)--(5,5));
draw((20,5)--(25,0));

\textbf{(A)}\ \frac18 \textbf{(B)}\ \frac16 \textbf{(C)}\ \frac15 \textbf{(D)}\ \frac14 \textbf{(E)}\ \frac13