Puzzles
Two ropes, 45 minutes
You have two ropes and a lighter. Each rope takes exactly one hour to burn, but they burn unevenly — half the rope might take five minutes or fifty. How do you measure exactly 45 minutes?
Reveal the answer
Light rope A at both ends and rope B at one end, all at once. A burns out in 30 minutes (two flames meet, however uneven the rope). At that moment, light B's other end — its remaining hour of rope now burns from both sides in 15 minutes. 30 + 15 = 45.
— Peter Winkler, Mathematical Puzzles: A Connoisseur's Collection — 2004 (classic puzzle, author unknown)
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