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November 18, 2017, 01:34:27 PM
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: What's the train speed? (Infy)  ( 4139 )
« : May 17, 2007, 08:53:32 PM From Prateek»



What's the train speed? (Infy)


A man is 3/8's of the way across a train bridge, when he hears the whistle of an approaching train behind him. It turns out that he can run in either direction and just barely make it off the bridge before getting hit. If he is running at 15 mph, how fast is the train traveling? Assume the train travels at a constant speed, despite seeing you on the tracks.

Note: From a 7th grade algebra book.

 
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« #1 : June 12, 2007, 01:43:55 PM From Narender»

Around 24 m/h
« #2 : June 12, 2007, 07:24:37 PM From PSM»

yep ur right it is minimum 24mph and also it will depend on the length of the bridge.............
« #3 : August 24, 2007, 06:46:02 PM From Abhishek»

I think the answer is 60 mph.....

Explanation:  
Suppose the man turns back and manages to escape from the tunnel just when the train is entering the tunnel. This means that in the time the man takes to rum 3/8 of tunnel, the train reaches the entrance.

Now, lets suppose he keeps running forward. By the time he moves 3/8th of tunnel more(i.e. 3/8+3/8=3/4th of tunnel in total), the train will be at the entrance(by previous calculation). The man again manages to escape the tunnel just when the train leaves the tunnel. This means that the train covers the full tunnel in the same time the man covers only 1/4th of the tunnel. Obviously, the train must be moving at 4 times the speed of man.

The speed of man being given 15 mph, the speed of train comes out to be 60 mph.
« #4 : August 24, 2007, 08:40:51 PM From abhishek_akj»

what an excellent explanation.........
« #5 : September 27, 2007, 02:26:40 PM From vishwakarma.deepak»

nice way to slove...I took more time using alzebra...
u are right it's 60 mph.

Good man!


Deepak.
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