A 300 meter train takes 15 sec to cross a man who is walking in the same direction at the speed of 6 km/hr. The speed of the train is:

78 km/hr

Let the speed of the train be x km/hr

The relative speed of the train with respect to the man = (x - 6) km/hr

⇒ (x - 6) = \(\frac{300}{15}\) × \(\frac{18}{5}\) (converting m/s to km/hr by multiplying with x\(\frac{18}{5}\))

⇒ (x - 6) = 20 × \(\frac{18}{5}\)

⇒ x - 6 = 72 

x = 78

Ans = 78km/hr