If a 200 m long train crosses a platform of the same length as that of the train in 20 seconds, then the speed of the train is:

72 km/hr

Speed = \(\frac{ Distance}{ Time}\)

Speed = \(\frac{\text{200 + 200}}{ 20}\) m/s

Speed =  \(\frac{400}{ 20}\) m/s

Speed =  20 m/s

Speed in km/hr:

20 x \(\frac{18 }{ 5}\)

4 x 18 = 72 km/hr

*Note* 

In order to convert km/hr to m/s - multiply by  \(\frac{ 5}{ 18}\)

In order to convert m/s to kn/hr - multiply by  \(\frac{ 18}{ 5}\)