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 July 10th, 2018, 05:15 AM #1 Senior Member   Joined: Aug 2014 From: India Posts: 458 Thanks: 1 How much time would it cross.... A train running at 65km/hr crosses a tunnel that is double of its length in 1 min 30 sec. In how much time would it cross another train which is half its length and coming from the opposite direction at 32.5km/hr? I can follow this procedure: 3x/90 seconds = 65km/hr --> after finding x, the length of the train (3x/2)/k seconds = 97.5 km/hr --> I use this x to find k I get k is 30 sec. but my calculation get lengthy, is there any shortcut to do this problem? I mean are there any proportionality approach etc for this problem? Last edited by Ganesh Ujwal; July 10th, 2018 at 05:59 AM.
 July 10th, 2018, 06:16 AM #2 Senior Member   Joined: May 2016 From: USA Posts: 1,310 Thanks: 551 Hint 1: In rate, time, distance problems you need to have two numeric values for two of the variables to find the numeric value of the third. In the first part of the problem (the tunnel part) you can find x, which is a distance. In the second part of the problem, you need to find time. You need to solve for x before starting on the second part. Hint 2: With two objects moving relative to each other, it frequently helps to pretend that one is at rest and the other is moving at an adjusted rate.
July 10th, 2018, 07:11 AM   #3
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Quote:
 Originally Posted by JeffM1 .....
I am asking for easy and quick approach. Yours approach I know it before.

July 10th, 2018, 09:51 AM   #4
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Quote:
 Originally Posted by Ganesh Ujwal I am asking for easy and quick approach. Yours approach I know it before.
That is the easy and quick approach. There are other derivations but they all come down to simultaneously solving two equations in two unknowns. Can you be more specific and tell us the kind of solution you are looking for?

-Dan

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