Start both glasses as the egg goes in. When the 4 runs out, turn it over. When the 7 runs out, turn that over too. When the 4 runs out again, at 8 minutes, the 7 has one minute of sand in its bottom half, so turn it over and it empties at 9.
You need to boil an egg for exactly 9 minutes, but all you have is a 4-minute hourglass and a 7-minute hourglass. How do you time it?
Give me a hintThe hint
Two runs of the 4 make 8. You need one more minute, and the 7 can give it to you.
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Start both, and turn the 7 over at 8 minutes
Source: Traditional hourglass puzzle