If you have a 7-minute hourglass and an 11-minute hourglass, how can you boil an egg in exactly 15 minutes?
Answer: See below.
Explanation: 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.
1 start both timers
2. when the 7 min timer runs out, start the egg, there are 4 mins left in the 11 min timer
3. when those 4 mins run out restart the 11 min timer