The clue is not always in what moves. Sometimes it is in what remains calm. In this square, a coin falls into the water at a precise moment — and that moment has a signature.
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Riddle statement
In a square there are three fountains that shoot water at regular intervals. One does it every 4 minutes, another every 6 and another every 9. At 12:00, all three raised the jet at the same time.
Later, before 12:30, a visitor sees that only two fountains raise the jet at the same time. On a whim, he throws a coin into the water of the only fountain that, at that moment, remained calm.
Two minutes later, that same fountain raises the stream.
At what time did he throw the coin and in which fountain did it fall?
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Solution
Answer: the coin was tossed at 12:18, in the fountain that releases water every 4 minutes.
The key is in the moments when two fountains activate together and one remains calm.
The 4-minute fountain and the 6-minute fountain matches every 12 minutes: it happens at 12:12 and 12:24. In both cases, the calm source is the 9-minute source.
The 6-minute source and the 9-minute source matches every 18 minutes: it occurs at 12:18. The calm fountain is the 4-minute fountain.
The 4-minute fountain and the 9-minute fountain do not matches again before 12:30 — their next meeting would be at 12:36.
Now the decisive fact comes in: two minutes after the coin falls, that same fountain raises the stream.
If the coin had fallen in the 9-minute fountain —calm at 12:12 or 12:24—the next activation of that source would not have arrived two minutes later. Instead, the 4-minute source, calm at 12:18, activates again at 12:20: exactly two minutes later.
Everything fits into a single possible moment.