A person standing on a railway platform noticed that a train took 21 seconds to completely pass through the platform which was 84 m long and it took 9 seconds in passing him. The speed of the train was

Let the train’s length be L m.

$$\frac{L}{9} = \frac{L + 84}{21}$$

21L = 9L +756

12L = 756

L = 63 m

$$\frac{63}{9}$$ =7 m/s = $$7 \times \frac{18}{5}$$

Speed = $$\frac{63}{9}$$ =7 m/s = $$7 \times \frac{18}{5}$$ = 25.2 Km/hr.