You have 2 hourglasses: 7-min and 11-min. How to time exactly 15 minutes?
Start both, flip 7
The answer
Start both. When 7 runs out (7 min), flip it. When 11 runs out (11 min), flip 7 again. 7 has 3 min left → 11+4... Actually: start both. At 7, flip 7. At 11, flip 7 (has 4 left, flip = 4 min). 11+4=15 min.
Source: Verified
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