You have two identical hourglasses: one measures exactly 7 minutes and the other measures exactly 4 minutes. Without bending, breaking, or timing with another device, how can you measure exactly 9 minutes using both hourglasses?
Start both the 7-minute and 4-minute hourglasses. When the 4-minute glass empties, flip it. When the 7-minute glass empties, 7 minutes have passed and 3 minutes have accumulated in the 4-minute glass. Flip the 4-minute glass again so those 3 minutes run back down, adding 3 to 7 to make 10? Let's use 9: Start both. When 4 runs out, flip it. When 7 runs out, 7 minutes have passed, 4-minute glass has 3 minutes at the bottom. Flip the 4-minute glass again? Let's keep it simple.
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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