There are two hourglasses - one for 7 minutes and one for 11 minutes. An egg needs to boil for 15 minutes. How can you measure this time using the available hourglasses?

#
With the help of two different hourglasses, you can measure not only the time equal to their "sum," but also the time equal to their "difference."

## Solution

"Start" both hourglasses simultaneously. When the 7-minute hourglass runs out, flip it and let it run for 4 more minutes until the 11-minute hourglass runs out. If you now flip the 7-minute hourglass again, it will run for exactly 4 more minutes, and in total, the hourglasses have been running for 15 minutes.