A train 125 m. in length, moves at a speed of 82 km/hr. In what time the train will cross a boy who is walking at 8 km./hr. in opposite direction?

When two objects move in opposite directions, their relative speed is the sum of their individual speeds. The train's speed is 82 km/hr and the boy walks at 8 km/hr in opposite direction, so relative speed = 82 + 8 = 90 km/hr = 25 m/s. Time to cross = distance / relative speed = 125/25 = 5 seconds. The boy's motion is opposite to the train, so they approach each other faster than if the boy were stationary.