A 400 m long train passes a man walking towards it at 6 km/h in 20 seconds. What is the speed of the train?

Explanation

1. 01

   Relative speed $= \frac{400}{20} = 20$ $m/s = 72$ km/h.

2. 02

   The man walks towards the train, so speeds add: train = $72 - 6 = 66$ km/h.