You possess two accurate sand timers: one measures 7 minutes and the other measures 11 minutes. What is the minimum elapsed real-world time needed to measure an interval of exactly 15 minutes?
15 minutes
Start both timers at minute 0. When the 7-minute timer finishes at minute 7, exactly 4 minutes remain in the 11-minute timer. Flip the 7-minute timer immediately. When the 11-minute timer runs out at minute 11, exactly 4 minutes of sand have fallen into the bottom of the 7-minute timer. Immediately flip the 7-minute timer back so those 4 minutes run back down. When it empties, 11 + 4 = 15 minutes have passed.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Hard. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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