A merchant has two hourglasses: a 7-minute glass and an 11-minute glass. He needs to time exactly 15 minutes to boil an egg, with no extra measuring tools. How can he do this?
Start both hourglasses simultaneously. When the 7-minute glass runs out, flip it immediately. When the 11-minute glass runs out, flip the 7-minute glass again (which now has 4 minutes accumulated). When that 7-minute run finishes, exactly 15 minutes have passed.
11 minutes pass when the 11-minute glass empties. At that exact moment, the 7-minute glass has run out once and run for 4 more minutes (total 11). Flipping it then gives 4 minutes. 11 + 4 = 15 minutes.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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