Four minute hourglass

Question:

A cook in a restaurant has a four minute hourglass, and a seven minute hourglass, made of sand that shows the exact time. A customer orders a nine-minute egg. Using the two glasses, how to cook exactly in the time given, not to a difference of even few seconds?

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Answer: Flip both hourglasses and start cooking. As the four minute hourglass runs out (4) flip it back and continue till the seven minute hourglass runs out (7) and twist it back. One minute later, the four minute hourglass runs out (8). At this point, flip the seven minute hourglass back over. The seven minute hourglass was running only for a minute, so when it is flipped over again it will run only for a minute (9) more. Total is nine minutes.

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