A moving train starts crossing a post, a tree, a pillar, a standing man or any object of negligible length/width when the front part of the train meets the object.

The crossing ends when the end or rear part of the train just leaves the post or pillar or whatever.

A moving train starts crossing a bridge, a platform or any object, whose length/width can be determined, when the front part of the train just meets the beginning of the bridge or platform.

The crossing ends when the end or rear part of the train just leaves the end of the bridge or platform or whatever object of determinable length.

Thus, to cross a post, a pillar, a tree or a standing man, a train will have to travel a distance equal to its own length.

Again, to cross a platform, a bridge or any object whose length/width can be determined, a train will have to cover a distance equal to the sum of its own length and the length of the bridge or platform.

In 1 hour or 60 minutes the train covers 72 km.

Therefore, in 1 minute the train covers 72/60 km.

So, in 1 second the train covers 72/60 ÷ 60 km = 72/60 x 1/60 km

So, in 18 seconds the train covers 72/60 x 1/60 x 18 km = 0.36 km = (0.36 x 1000) metres = 360 metres.

Again, in 18 seconds the train travels 360 metres.

So, in 1 second the train travels 360/18 metres.

Therefore, in 30 seconds the train travels 360/18 x 30 metres = 600 metres.

According to the rule, in 30 seconds the train travels a distance equal to the sum of its own length and that of the platform. This sum is 600 metres.

Now to get the length of the platform we have to subtract the length of the train from the sum.

The crossing ends when the end or rear part of the train just leaves the post or pillar or whatever.

A moving train starts crossing a bridge, a platform or any object, whose length/width can be determined, when the front part of the train just meets the beginning of the bridge or platform.

The crossing ends when the end or rear part of the train just leaves the end of the bridge or platform or whatever object of determinable length.

Thus, to cross a post, a pillar, a tree or a standing man, a train will have to travel a distance equal to its own length.

Again, to cross a platform, a bridge or any object whose length/width can be determined, a train will have to cover a distance equal to the sum of its own length and the length of the bridge or platform.

**Problem:**A train travelling at a speed of 72 km per hour passes a man standing on a platform in 18 seconds. The train passes the platform in 30 seconds. Find the length of the train and that of the platform.**Solution:**According to the rule, in 18 seconds the train covers a distance equal to its own length as it takes 18 seconds to pass the standing man.In 1 hour or 60 minutes the train covers 72 km.

Therefore, in 1 minute the train covers 72/60 km.

So, in 1 second the train covers 72/60 ÷ 60 km = 72/60 x 1/60 km

So, in 18 seconds the train covers 72/60 x 1/60 x 18 km = 0.36 km = (0.36 x 1000) metres = 360 metres.

**Therefore, the length of the train is 360 metres.**Again, in 18 seconds the train travels 360 metres.

So, in 1 second the train travels 360/18 metres.

Therefore, in 30 seconds the train travels 360/18 x 30 metres = 600 metres.

According to the rule, in 30 seconds the train travels a distance equal to the sum of its own length and that of the platform. This sum is 600 metres.

Now to get the length of the platform we have to subtract the length of the train from the sum.

**Therefore, the length of the platform is (600 - 360) metres = 240 metres.**
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