On The Autobahn!
He maintained a constant speed and as an overtaking car went past him he started counting the seconds until that car would reach a milestone such as a sign, a clump of trees, or some marker.(call this count "a"). Then he continued counting until he reached that same spot. (call this count "b")
If his constant speed was 120 km/hr, a = 17 s, and b = 29 s. what was his estimate of speed of the passing car? (to nearest integer)