Posted by **Simran kaur** on Saturday, September 8, 2012 at 3:48am.

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.

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**MathMate**, Saturday, September 8, 2012 at 10:01am
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) lamp-post.

Can you work out the length of the fast train?

Hint: It's more than half a km long.

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**Simran kaur**, Saturday, September 8, 2012 at 12:09pm
Sory i cnt find length plz give me solution..

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**MathMate**, Saturday, September 8, 2012 at 4:45pm
Hint:

Distance = speed * time

They have to have consistent units.

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**Simran kaur**, Sunday, September 9, 2012 at 2:00am
I solved it my ans is630m. Bt it is wröng book ans. Is 315m plz give complt solution fast..

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**MathMate**, Sunday, September 9, 2012 at 5:07am
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.

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**Simran kaur**, Sunday, September 9, 2012 at 10:06am
Thnx a lot..

- Math :) -
**MathMate**, Sunday, September 9, 2012 at 10:22am
You're welcome!

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