rupali sehgal
Last Activity: 12 Years ago
working in ref frame attached to 2,

velocity of 1 wrt 2= v1+v2
let length of train be x metres each.
therefore time taken to cross each other= 2x/v1+v2=3
=> 2x= 3v1+3v2----------------------------------------------(1)
Case 2:
velocity of 1= v1+50v1/100=3v1/2
now, time taken to cross each other= 2x -------------- = 2.5
3v1/2- v2
=> 2x= 2.5v2+ 7.5v1/2---------------(2)
equating (1) and (2)
3v1+3v2= 2.5v2+7.5v1/2
=> 1.5v1/2= v2--------------------------------(3)
now if they move in same direction:

velocity of 1 wrt 2= v1-v2
time taken to cross each other= 2x/v1-v2
= 2x/v1-0.75v1 (from 3)
= 2x/0.25v1
= 3v1+3 v2/ 0.25v1 (from 1)
= 3v1+ 0.75v1/0.25v1 (from 3)
= 3.75v1/0.25v1
= 15 sec
all the best!