Moscow Anthology/Amusing Problems/1

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Two school students are observing a steam locomotive, the smoke from whose smokestack appears to be rising in a vertical plume.
As they have been taking meteorological measurements, they know that the wind is steady and gust-free, and its speed is $20$ miles per hour.

How do they know the train's speed?


As the smoke was rising vertically upwards, the speed of the smoke must have been $0$ relative to the wind.

Hence the smoke was travelling at the same speed as the wind.

Thus it must have been rising from a smokestack that is travelling at that same speed.

It follows that the train itself was travelling at $20$ miles per hour.