Hourglasses 1 Puzzle - Solution

The Puzzle:

Hourglasses 1
You need to boil eggs for exactly 9 minutes, or else the visiting Duchess will complain, and you will lose your job as head chef.

But you have only 2 Hourglasses, one measures 7 minutes, and the other measures 4 minutes. How can you correctly measure 9 minutes?

Our Solution:

Put the eggs on to boil and start both hourglasses running.

When the 4-minute one runs out, turn it over immediately so it starts counting 4 minutes again

When the 7-minute one runs out, turn it over so it starts counting 7 minutes again

The moment the 4-minute one runs out for the second time, turn the 7 minute hourglass over - it will have only been running exactly one minute.

Let the sand run back again (1 minute more) and then take the eggs off straight away, because they will have boiled for 9 minutes.

(4 minutes twice, plus one more minute = 9 minutes!)
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