A passenger in a train with speed 72 km/h observed another train coming from the opposite direction with speed 32.4 km/h. What's the length of the second train if it crosses the passenger in 10s?

Let an Observer is in a train that moves with speed 72 km/hr.

If this observer observe a train in opposite direction that moves with actual speed u km/hr ( not as oberved by observer ),

then speed of this train as observed by the oberver is ( 72 + u ) km/hr .

 

Since speed observed by observer is 32.4 km/hr , which is less than observer train's speed ,

observed train in opposite direction is moving in same direction as the direction of observer's train. 

 

Hence when observer's train crosses the train in opposite direction ,

relative velocity between two train is 32.4 km/hr = 32.4 × (5/18) = 9 m/s

 

if time taken for train  is 10 s for crossing the passenger (or observer ) , length of train = 9 × 10 = 90 m