Insight · ★★☆☆☆ · ~4 min
The Two Trains & the Fly
A fly bounces between two closing trains forever. How far does it fly?
by MindYourPuzzle
The puzzle that produced the most famous one-liner in mathematics.
Two trains are 100 miles apart on the same track, heading straight toward each other. Each travels at 50 mph.
At the instant they start, a fly takes off from the front of one train, flying at 75 mph. It reaches the other train, instantly turns around, flies back to the first, turns around again — bouncing between the two trains, over shorter and shorter distances, until the trains collide and the fly is (sadly) squashed.
What total distance does the fly travel before the trains meet?
(Answer in miles.)
miles
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