Question

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

Answer 1

Start both hourglasses. At the end of 7 minutes restart the small one. At the end of 11 minutes restart the big hourglass. The small one at this point has 3 minutes to go. When it empties begin boiling your eggs. The big hourglass will run for another 8 minutes. After that use the small hourglass to measure another 7 minutes. 8 + 7 = 15. This solution requires turning the small hourglass twice and the big one just once which sums up to 3 turns.

Step-by-step explanation:

Answer 2

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.