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September 26th, 2018, 09:30 PM   #1
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I got three different answers for this problem

The ratio of the speeds of a goods train and a passenger train is 3:7. The two trains can cross each other in 40 sec. A man on the passenger train observes that the goods train crosses him in 25 seconds. If the goods train is 275m long, what is the length of the passenger train?

A)

First method:
Crossing each other means travelling in opposite direction:
(275 + X)/275 = 40/25 = 8/5
It can written as (5+3)/5 = (275 + X)/275
Here 5 corresponds to 275
3 corresponds to 275/5*3 = 165

Second method:
Total distance = 275 + X
X means length of passenger train.
Relative speed = either 7 + 3 = 10 (or) 7 - 3 = 4.
There are 2 relative speed values because they didn’t mention whether trains are approaching each other or moving in other direction:
Time = (Distance )/speed
40 sec = (275 + X)/10 or (275 + X)/4
X = 400 – 275 (or) 160 – 275
= 125 (or) - 115

Which is the correct one?

Last edited by skipjack; September 27th, 2018 at 01:55 PM. Reason: to correct typos
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September 27th, 2018, 01:12 PM   #2
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Go read up a bit:

https://www.indiabix.com/aptitude/pr...-trains/038003
See #11
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Last edited by Denis; September 27th, 2018 at 01:42 PM.
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September 27th, 2018, 02:10 PM   #3
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The first method is correct.

The second method is incorrect.

The trains' directions are irrelevant. If u m/s is the relative speed, 40 = (275 + X)/u.

You didn't use the man's observation, which gives 25 = 275/u, so u = 11. Hence X = 165.
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September 27th, 2018, 02:15 PM   #4
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Quote:
Originally Posted by skipjack View Post
If u m/s is the relative speed, 40 = (275 + X)/u.
Thanks for using "u", my favorite variable
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