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Simple trains problem (Posted on 2017-08-29) Difficulty: 2 of 5
A freight train going in one direction takes 3 minutes to pass a crossing. A slower train of the same length on a parallel track in the other direction takes 4 minutes.

How long does it take these trains to pass each other?

No Solution Yet Submitted by Jer    
Rating: 3.0000 (1 votes)

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Solution or, in general Comment 2 of 2 |

Or in general if the two times are “a” and “b”, and the train length is “L”, the two velocities are L/a and L/b. 

Consider the frame of reference to be one of the trains, as if one train was standing still.  The desired time is the time it takes to go a distance of 2L with a combined velocity of L/a + L/b

Time:  2ab / (a+b)

e.g. 2*3*4/7 = 3 3/7 as Steve Herman has posted.
  Posted by Larry on 2017-08-29 10:25:39
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