You are a cook in a remote area with no clocks or other way of keeping time other than a four-minute and a seven-minute hourglass. On the stove is a pot of boiling water. Jill asks you for a nine-minute egg, and you know she is a perfectionist and can tell if you undercook or overcook the egg by even a few seconds. How can you cook the egg for exactly 9 minutes?
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