Teaser Of The Day No. 233

[Carl]

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


Answer will be provided next Thursday evening after 21:00.

[Brandyboy]

Start the 2 hourglasses together.
When the 7 minute hourglass finishes, turn it over.
When the 11 minute hourglass finishes, turn the 7 minute hourglass over again.
When the 7 minute hourglass finishes the 15 minutes will be up.

[Carl]

ANSWER

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

1. Start both hourglasses as you start boiling the egg.
2. After the 7-minute hourglass runs out, turn it over 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.