Burning ropes
You have two ropes and a lighter. Each rope takes exactly 60 minutes to burn from one end to the other, but they burn unevenly, so half a rope does not take 30 minutes.
How do you measure exactly 45 minutes?
Show hint
Lighting a rope at both ends makes it burn out in exactly half the time, however uneven it is.
Show solution
- Light rope A at both ends and rope B at one end, at the same time.
- Rope A burns out after exactly 30 minutes. Rope B now has 30 minutes left.
- Light the other end of rope B. It burns out in 15 more minutes.
30 + 15 = 45 minutes.
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