Иван Сергейитж Ракнэр must be new here...
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Another solution is: spoiler * Start both hourglasses at the same time * Wait until the seven minute hourglass is over. You won't need it anymore. * This leaves (11-7) 4 minutes in the eleven minute hourglass * Put the egg in the boiling water * When the remaining 4 minutes have past, flip the eleven minute hourglass * This adds 11 minutes to the 4 minutes that have just passed, 11+4 = 15 * When the hourglass finishes the seconds time, the egg boiled for 15 minutes
This has an overhead of 7 minutes (see point 1 and 2) ... which one could use to get the water to boil ;)context