A boy sitting in a train which is travelling at 25 km/hr observes that a goods train travelling in opposite direction, takes 4.5 seconds to pass him. If the train is 140 m long, find its speed. A) 77km/hr B) 79km/hr C) 87km/hr D) 89km/hr
ANSWER : C
Explanation:
Relative speed = distance / time
= (140 / 4.5) m/s
= 31.11 m/s
Convert m/s into km/hr
31.11 m/s = (31.11 * 18 / 50 km/hr
= (560 / 5) km/hr
= 112 km/hr
Relative speed = speed of train + speed of train (by sitting)
speed of train = Relative speed - speed of train (by sitting)
= (112 – 25) km/hr
= 87 km/hr
Therefore the speed of goods train is 87 km/hr.