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

Answer: (a)

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.