2001 AMC 10 Problem 15 This problem is from the 2001 AMC 10. It is Problem 15.

A street has parallel curbs $ 40 $ feet apart. A crosswalk bounded by two parallel stripes crosses the street at an angle. The length of the curb between the stripes is $ 15 $ feet and each stripe is $ 50 $ feet long. Find the distance, in feet, between the stripes.
  $ \textbf{(A)}\ 9 \qquad \textbf{(B)}\ 10 \qquad \textbf{(C)}\ 12 \qquad \textbf{(D)}\ 15 \qquad \textbf{(E)}\ 25 $

What is the difference between the distance between the stripes and the length of the curb between the stripes? My diagram: 



The distance between the strips is the distance between those parallel lines and the length of the curb is the distance which is measured through the given line i.e. the road.
$\triangle ABE$ is similar to $\triangle CDE$
$AB$ is the distance between strips.
Therefore,
$\displaystyle \ \ \ \ \ \frac{CD}{AB}=\frac{CE}{AE}$
$\displaystyle \Rightarrow\frac{40}{AB}=\frac{50}{15}$
$\displaystyle \Rightarrow AB=12$
