Question

A man standing on a platform observes a train going in one direction takes 4 seconds to pass him. Another train of the same length going in the opposite direction takes 5 seconds to pass him. What would be the time taken by the two trains to cross each other?

Answer 1

Solution 40 seconds
because
s= d/t
let s1 be the speed of 1st train  = d/4
let s2 be the speed of 2nd train = d/5


relative speed in same direction s= s1 - s2
                                                 = ( d/4 - d/5 )
                                                 = d/20

now t = d/s ( time taken to cross each other moving in same direction  so d= 2d because 2 trains of same length d )

    t =  $\frac{2d}{ \frac{d}{20} }$
     
      = $\frac{2d X 20}{d}$     
     
       =40 sec

Thus the time taken to cross would be 40 seconds

Answer 2

the time taken for the trains to cross each other is opposite direction must be 40/9 seconds