A train is moving at a speed of 70m/s. It takes 2 seconds to cross a jeep that is travelling in the opposite direction at a speed of 31m/s. What is the length of the train?
The distance to be covered is equal to the length of the train. This distance has to be covered at a net relative speed equal to the sums of the speeds of the jeep and the train, so v = 70 + 31 = 101. The length of the train will be time × speed = $2 × 101$ = 202meters.