Using two hourglasses of 7 and 11 minutes,
How can we calculate the fifteen minutes it takes to cook the egg in the shortest possible time?
The two clocks of 7 and 11 minutes are counted at the same time that we lay the egg in the casserole. When the sand is finished in the 7-minute clock, we turn it around and wait for the 11-hour clock to run out. At that time, 4 minutes of the 7-hour clock will have elapsed, so if we turn it over it will count the 4 minutes missing to cook the egg. In total 11 + 4 = 15 minutes will have elapsed.