We received the order to cook an egg for exactly 9 minutes. For this we have an egg, a pan full of boiling water on the fire and two hourglasses of 4 minutes and 7 minutes respectively.
How can we calculate exactly the 9 minutes?
We put the egg in the pan and turn the hourglasses to start counting the time. At the end of the 4 minute clock we turn it over. After 7 minutes the other clock will be over and we turn it over. Soon, the 4-minute clock will be over again. At that time it will have been 8 minutes and we will turn both watches. Since only one minute of the 7-minute clock will have passed, when all the sand on that clock has just fallen, exactly 9 minutes will have passed and we can remove the egg from the casserole.