Needing to boil an egg for exactly 15 minutes I stumbled through my egg timers until I found  the other two that were unbroken. 7 minutes and 11 minutes where the only ones I could use. How on earth could I cook an egg for exactly 15 minutes?

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Start both hourglasses.
When the 7 minute glass runs out, turn it over.
When the 11 minute glass runs out, turn the 7 minute glass over again.
When the 7 minute glass runs out your egg will be ready.

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