If you have a 7-minute hourglass and an 11-minute hourglass, how can you boil an egg in exactly 15 minutes?

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.