Flip both hourglasses. Once the 7 minute hourglass runs out, stop the 11 minute hourglass by turning it on it’s side. There are now 4 minutes remaining in the 11 minute glass. Now flip both glasses again and resume until the 11 minute hourglass runs out. At that point, stop the 7 minute hourglass by turning it on it’s side. There is now 4 minutes in the 7 minute hourglass. Now you can restart the 11 minute hourglass in it’s entirety and then follow it with the remaining 4 minutes left in the 7 minute hourglass, adding up to a total of 15 minutes.
Published by: Jetta Moon