A train 150 metre long passes a standing man in 15 second. While it crosses another train of the same length coming from opposite direction in 12 seconds. Find the speed of the second train.

First train's speed = 150m/15s = 10 m/s. When trains cross in opposite directions, relative speed = sum of speeds. Total distance = 150 + 150 = 300m. Time = 12s, so relative speed = 300/12 = 25 m/s. Second train's speed = 25 - 10 = 15 m/s = 15 × (18/5) = 54 km/hr. The key is understanding relative speed in opposite directions.