Answer
To boil the egg in exactly 15 minutes, follow these steps.
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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