Explanation:
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 = 5/18 * 72 = 20 m/sec
Therefore, length of the platform = 20 * 12 = 240 meters.
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