A Moscow Puzzle
Two children, a boy and a girl, are sitting near a railroad on a hill taking some meteorological measurements. The wind is blowing constantly at 20 mph. From their viewpoint, the children can see the two train stations of Brilliant Town and Moscowville below. A train passes between these two stations, 5 miles apart. The girl is very smart, but not the boy. They do not have a timer, yet the girl correctly guesses the train's speed. Given that the smoke drifting from the engine rises up in a straight line, can you too guess the train's speed, in mph?
Adapted from the Moscow Puzzles by Boris Kordemsky.
The answer is 20.
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If the smoke goes straight up then the wind must be blowing at the same speed of the train. Wind speed = Train speed = 2 0 .