Given: Length of train A = 121 m Length of train B = 129 m Speed of train A = 40 km/h Speed of train B = 68 km/h Formula used: Time = Distance/Speed Calculation: Length of train A and B =121+129=250 m Relative speed = (40 + 68) = 108 km/h = 108 × (5/18) m/sec = 30 m/sec Now, Time = 250/30 = 8.33 seconds