2001 AMC 10 Problem 15

The crosswalk is a parallelogram. Using the curb ($15$ ft) as base and the street width ($40$ ft) as height, its area is $15\cdot40=600$ square feet. Using a stripe ($50$ ft) as base, the area equals $50$ times the distance $d$ between the stripes, so $d=\frac{600}{50}=12.$ Thus, the correct answer is C.