Posted by Simran kaur on .
2trains of the same length were running in opposite direction at 54km/hr and 72km/hr respectively.find the length of each train if the faste train passed a man in the slower train in 18seconds.

Math 
MathMate,
The relative velocity between the two trains is 54+72=126 km/hr.
The 18 seconds would be equivalent to the time it takes a train at 126 km/hr to pass a (stationary) lamppost.
Can you work out the length of the fast train?
Hint: It's more than half a km long. 
Math 
Simran kaur,
Sory i cnt find length plz give me solution..

Math 
MathMate,
Hint:
Distance = speed * time
They have to have consistent units. 
Math 
Simran kaur,
I solved it my ans is630m. Bt it is wr0‹2ng book ans. Is 315m plz give complt solution fast..

Math 
MathMate,
We will assume that train A goes west at 72 km/h, and train B goes east at 54 km/h.
We further assume that at time t=0, the two trains meat head to head, with the stationary observer at the head of train B, i.e. the engineer of train B.
In 18 seconds, train B would have gone east by 54*(18/3600)=270 m while train A would have gone west by 72*(18/3600)=360 m.
That would give a total length of train A of 270+360=630 m. as you had it.
IF the question had been
"if it took 18 seconds for the trains to clear each other..."
then the length of each train would be 315 m, exactly half of 630 because it takes double the time for the trains to clear each other.
Unfortunately, I believe the answer 315m does not correspond to the question you're working on. 
Math 
Simran kaur,
Thnx a lot..

Math :) 
MathMate,
You're welcome!