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Puzzle No. 17

Posted on 22 April 2016

The town hall contains 3 bells: the first one rings once every 6 minutes, the second rings once every 15 minutes, and the third one rings once every 21 minutes. If the bells rang together for the first time at midnight, then what is the shortest time interval (in minutes) during the rest of the day, for which no bells were ringing?

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