Riddles about eggs

logicmath

You have two sand hourglasses, one that measures exactly 4 minutes and one that measures exactly 7 minutes. You need to measure out exactly 2 minutes to boil an egg. Using only these two hourglasses, how can you measure out exactly 2 minutes to boil your egg?
Flip over both hourglasses at the same time. 1. After 4 minutes, the 4-minute hourglass will be done, and there will be 3 minutes left in the 7-minute hourglass. Immediately flip the 4-minute hourglass over again. 2. After 3 more minutes, the 7-minute hourglass will be done, and there will be exactly 1 minute left in the 4-minute hourglass. Immediately flip the 7-minute hourglass over again. 3. After 1 more minute, the 4-minute hourglass will be done again, and there will be exactly 6 minutes left in the 7-minute hourglass. Immediately flip over the 4-minute hourglass. 4. After 4 more minutes, the 4-minute hourglass will be done again, and there will be exactly 2 minutes left in the 7-minute hourglass. At this point, put your egg in the boiling water. When the 7-minute hourglass is done, it will have been exactly 2 more minutes, and your egg will have boiled just right. Or after step 2 just flip 7-minute hourglass for second minute.
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