JEE Class main Answered

As we seen from figure, if a train of length 245 m is crossing the platform of length 490 m ( double the length of train) ,

then distance moved by train is ( 245+490 ) m = 735 m .

If the train moves this distance in 45 s , then speed of train = 735 m / 45 s = ( 49/3 ) m/s

Speed of second train = 63 km/hr = 63 × (5/18)  = 35/2  m/s

If both the trains are moving , then relative speed of one train with respect to other is  [ (49/3) + (35/2) ] m/s = 203/6 m/s .

As seen from figure, after crossing each other , distance moved by each train is 609 m ( sum of their lengths )

Time taken by each train to cross each other = distance / speed = 609 / ( 203 / 6 ) s = 18 s