Hourglass

Question:

If you have an 11 minute and 13 minute hourglass, how can you accurately time 15 minutes?

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Answer: Start both hourglasses then when the 11 minute hourglass has finished immediately flip it again. When the 13 minute hourglass runs out the 11 minute hourglass will have 9 minutes left, so flip it and it will last another 2 minutes, 13 minutes + 2 minutes = 15 minutes.

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