A wizard brews a potion using two hourglasses to time a 9-minute spell. One hourglass runs out in 4 minutes, and the other runs out in 7 minutes. How can the wizard measure exactly 9 minutes using only these two hourglasses?
Start both at the same time. When the 4-minute glass empties, flip it immediately. When the 7-minute glass empties, flip it too. When the 4-minute glass empties the second time, exactly 9 minutes have passed.
At 4 minutes, the 4-min glass is flipped. At 7 minutes, the 7-min glass is flipped (3 minutes of sand have passed through it). At 7 minutes + 2 minutes (when the flipped 4-min glass empties), total time is 9 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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