When a train passes a pole it essentially travels a distance equal to its own length. Given, train, 200 m long, passes a pole in 20 seconds. Speed of this train = 200/20 = 10 m/s Also, when a train crosses another train, it travels a distance equal to the sum of lengths of both the trains, at relative speed. Given, it crosses another train of the same length travelling in the opposite directions in 10 seconds. Let the speed of 2nd train be a. Relative speed = (10 + a) m/s Total distance = 200 + 200 = 400 m ∴ 10 + a = 400/10 ⇒ a = 30 m/s