With a seven-minute hourglass and an eleven-minute hourglass, what is the quickest way to time the boiling of an egg for fifteen minutes?
Start the 7 and 11 minute hourglasses when the egg is dropped into the boiling water. When the sand stops running in the 7-glass, turn it over. When the sand stops in the 11-glass, turn the 7-glass over again. When the sand stops in the 7-glass, 15 minutes will have elapsed.