#4. How can you use a 7 minute hourglass and an 11 minute hourglass to time 15 minutes?

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1. Start both hourglasses as you start boiling the egg.
2. After the 7-minute hourglass runs out, turn it to start it again.
3. Four minutes later, when the 11-minute hourglass runs out, turn the 7-minute hourglass again.
4. Wait for the 7-minute hourglass to run out, which will take another four minutes and get you to exactly 15 minutes of boiling time.

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