Bunuel wrote:
How much time will an express train of length 150 meters and running at a speed of 75 kmph take to cross a man walking at 5 m/s inside a passenger train of length 200 meters and running at 15 kmph in a direction opposite to that of the express train. The man is walking inside the train and in opposite direction to that of the train. (Assume the man does not reach the end of his train meanwhile.)
A. 5 sec
B. 6 sec
C. 7.5 sec
D. 8 sec
E. 10 sec
The speed of the express train is the opposite of that of the passenger train and the man walking inside the passenger train.
Therefore the other two speeds are working against the express train. We can add all the speeds together and treat the entire system as one object. Hence treating the man and passenger train as stationary, the express train has a speed of:
75 km/h + 15 km/h + 5 m/s = 90 km/h + 5 m/s = 90 * 5/18 m/s + 5 m/s = 30 m/s.
The total distance we need to travel however is only 150 meters since the express train is only 150 meters long (or think of it as the time needed for the man to pass the 150-meter express train). Therefore 150 m / (30 m/s) = 5s is the answer.
Ans: A
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