GMAT MATH - Speed, Time and Distance

May 4, 2004

A train traveling at 72 kmph crosses a platform in 30 seconds and a man standing on the platform in 18 seconds. What is the length of the platform in meters?

(A)	240 meters
(B)	360 meters
(C)	420 meters
(D)	600 meters
(E)	Cannot be determined

Correct Choice (A) Answer (240 meters)

Explanatory Answer

When the train crosses a man standing on a platform, the distance covered by the train is equal to the length of the train.

However, when the same train crosses a platform, the distance covered by the train is equal to the length of the train plus the length of the platform.

The extra time that the train takes when crossing the platform is on account of the extra distance that it has to cover = length of the platform.

Therefore, length of the platform = speed of train * extra time taken to cross the platform

Length of platform = 72 kmph * 12 seconds

Converting 72 kmph into m/sec, we get 72 kmph = = 20 m/sec

Therefore, length of the platform = 20 * 12 = 240 meters.