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Fifteen minutes with a 7 and an 11 minute glass

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You have two hourglasses: one runs for exactly 7 minutes, the other for exactly 11 minutes. A dish has to bake for exactly 15 minutes, and it must go into the oven at the same moment you start timing; you cannot "get ready" first.

How do you time it? Flipping a glass is instantaneous and can be done at any moment, including when a glass has run only partway.

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Eleven plus four is fifteen. Can you make the 7-glass show you exactly four minutes at the right moment?

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