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Problems on Trains - General Aptitude

Q1:

A train travelling at 48 kmph completely crosses another train having half its length and travelling in opposite direction at 42 kmph, in 12 seconds. It also passes a railway platform in 45 seconds. The length of the platform is

A 400 m

B 450 m

C 560 m

D 600 m

ANS:A - 400 m

Let the length of the first train be x metres.

Then, the length of the second train is ( x ( metres.
2
Relative speed = (48 + 42) kmph = ( 90 x 5 ( m/sec = 25 m/sec.
18
Therefore [x + (x/2)] = 12 or 3x = 300     or     x = 200.
25 2
Therefore Length of first train = 200 m. Let the length of platform be y metres.
Speed of the first train = ( 48 x 5 ( m/sec = 40 m/sec.
18 3
Therefore (200 + y) x 3 = 45
40
=> 600 + 3y = 1800 => y = 400 m.