Hourglasses

You have a 7-minute hourglass and an 11-minute hourglass. How can you measure exactly 15 minutes with them?

SOLUTION

Turn both hourglasses upside down. When the time of the 7-minute hourglass runs out, turn it upside down. When the time of the 11-minute hourglass runs out, turn the 7-minute hourglass upside down again. Finally, when the time of the 7-minute hourglass runs out, exactly 15 minutes will have been passed.

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