1.  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

Answer: Option A

Explanation:

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.