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a passenger train and a freight train

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a passenger train and a freight train

by hemant_rajput » Mon Feb 04, 2013 9:55 am
Q2 . Two trains, a passenger train and a freight train, are running in the same direction on parallel railway tracks . The passenger train takes 3 times as long to pass the freight train as when they are going in opposite direction.
If the trains run at uniform speeds with freight train running at a mx speed of 25 kmph, what is the speed of passenger train in kmph?
a. P = 50
b. p<=50(p>0)
c. 0<=p<=50
d. 20<=p<=50
e. 0<=p<=20


I got the p = 50, but the OA says b. I don't know how to deduce the value less than 50; any help is appreciated.
I'm no expert, just trying to work on my skills. If I've made any mistakes please bear with me.
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by vkb001 » Tue Feb 05, 2013 5:24 pm
The question doesn't say how fast the passenger train (PT) is relative to the freight train (FT). It could be slower, or faster.

So, if the PT is slower, you'd get a negative relative speed w.r.t. the FT (i.e. p-25 < 0).
If PT is same or faster, the relative speed w.r.t. the FT would be positive (i.e. p-25 >= 0).

Depending on the case, the speed of the PT would be different, hence (b) is the right answer.

(c) is wrong, because it has p=0 as a possibility (which means that the PT is not moving).
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