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Cook the hard boiled egg II

Cook the hard boiled egg II

Using two hourglasses of 7 and 11 minutes,

How can we calculate the fifteen minutes it takes to cook an egg with the least number of turns of the clocks?

Solution

Both clocks are set in motion simultaneously and after 7 minutes we place the egg in the casserole. When the sand of the 11-minute clock runs out, we turn it over so that when the clock's sand has run out for the second time, 4 + 11 = 15 minutes of cooking will have elapsed having turned the clock once.