A fifteen-minute interval from two ropes

Question

You have two ropes and some matches. Each rope burns from one end to the other in exactly 60 minutes, but both burn unevenly and they are not identical. You cannot cut or mark them. Use the ropes to mark out an interval of exactly 15 minutes. The interval does not have to start the moment you strike the first match; it just has to be exactly fifteen minutes long, with a clear start and a clear end.

Accepted Answer

At time zero, light rope A at both ends and rope B at one end. When rope A burns out (30 minutes), light the other end of rope B. Rope B then burns out 15 minutes later. The interval from rope A burning out to rope B burning out is exactly 15 minutes. Step by step. - Rope A, lit at both ends, is fully consumed after 30 minutes, because two flames share its 60 minutes of burn. - During those 30 minutes rope B has been burning from one end, so it has exactly 30 minutes of burn left, wherever the flame happens to be along its length. - Lighting rope B's second end at that moment halves the remai…

A fifteen-minute interval from two ropes

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