A train 300 m long crossed a platform 900 m long in 1 minute 12 seconds. The speed of the train (in km/hr) is

60

Speed = \(\frac{ Distance}{ Time}\)

Speed = \(\frac{\text{300 + 900}}{ 72}\) as 1 min 12 seconds = 60 + 12 seconds = 72 seconds

Speed = \(\frac{\text{1200}}{ 72}\) 

Speed = \(\frac{\text{50}}{ 3}\) m/s

Speed in km/hr = \(\frac{\text{50}}{ 3}\) x \(\frac{18 }{5 }\)

Speed in km/hr = 60km/hr