Two trains of equal length travelling in opposite directions at 78 km/h & 117 km/h cross each other in 12 seconds . In how much time (in seconds) does the first train cross a platform of length 455 meter ?

36

Let the length of each train = x

Distance = Speed × Time

To convert km/hr into m/sec. 

 A km/hr  × \(\frac{5}{18}\)  = B m/sec     

As the trains travel in opposite direction.

So, their relative speed = ( 78 + 117 ) km/hr

 2x = (78 + 117) × \(\frac{5}{18}\) × 12

  2x = 650 m

  x = 325 m

  length of  ( Platform + Train )   =  Speed × Time

   (455 + 325) = 78 × \(\frac{5}{18}\) × T

    T = 36 seconds