You have two hourglasses: one measures 7 minutes and the other measures 4 minutes. You need to time an exact 9-minute boiling process for pasta. How do you do it using only these two hourglasses?
Start both hourglasses at the same time. When the 4-minute glass runs out, flip it immediately. When the 7-minute glass runs out, 7 minutes have passed and the 4-minute glass has 1 minute left. Flip the 7-minute glass instantly, and when the 4-minute glass finishes its remaining 1 minute, the 7-minute glass will have run for exactly 2 more minutes (7 + 2 = 9).
Total elapsed time equals the initial 7 minutes plus the 2 minutes elapsed on the 7-minute glass while finishing the leftover minute of the 4-minute glass.
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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