A train 936 meters long is moving at a speed of 72m/s. How long will it take to cross a man who is running in the opposite direction at a speed of 84m/s?
The distance to be covered is equal to the length of the train, so s = 936. This distance has to be covered at a net relative speed equal to the sums of the speeds of the man and the train, so v = 72 + 84 = 156. The time will be distance/speed = $936/156$ = 6 s.