A train, 100 m long travelling at 60 km/hr passes another train, twice as fast as this train and travelling in opposite direction, in 10 seconds. Find the length of the second train.

Train A: length = 100 m, speed = 60 km/hr = 16.67 m/s. Train B: speed = 120 km/hr = 33.33 m/s, opposite direction. Relative speed = 16.67 + 33.33 = 50 m/s. Time = 10 seconds. Total distance = relative speed × time = 50 × 10 = 500 m. Length of train B = total distance - length of train A = 500 - 100 = 400 m. Option B is correct.