A 450 meter train takes 30 sec to cross a man who is walking in the same direction at the speed of 3 km/hr. The speed of the train is:

57 km/hr

Let the speed of the train be x km/hr

The relative speed of the train with respect to the man = (x - 3) km/hr

⇒ (x - 3) = \(\frac{450}{30}\) × \(\frac{18}{5}\) (converting m/s to km/hr by multiplying with x\(\frac{18}{5}\))

⇒ (x - 3) = 15 × \(\frac{18}{5}\)

⇒ x - 3 = 54

x = 57

Ans = 57 km/hr