Since the 2 triangles are similar, their sides are proportional.
You also know AD is 20 yards, since it tells you that is the crosswalk length at A.
So the proportion is :
50 / 120 = 20 / BC
Or
if you flip it over,
120 / 50 = BC / 20
Then you can solve for BC.
120 / 50 = BC / 20
(20)(120/50) = BC
BC = 48
