Puzzle:

There are two hourglasses. The small one can measure 5 minutes and the large one can measure 7 minutes. How can we measure 16 minutes with 2 hourglasses running together? (If one of the hourglasses finishes leaking, then it must be flipped over immediately to keep it running. )


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Solution:

Start both hourglasses at the same time. You flip over the small hourglass when it finishes the first 5 minutes. Then you flip over the large one when it finishes its first 7 minutes. When the small hourglass runs to 10 minutes, flip over the small and large hourglass at the same time. It means the large hourglass will run 3 minutes again. When the large hourglass run to 13 minutes (7 + 3 + 3), flip both the small and large hourglass over again to run another 3 minutes.