Posted tagged ‘burning’

Burning ropes to measure time

December 15, 2007

(Can be found in Leino’s puzzle page)
You are given a box of matches and a piece of rope. The rope burns at the rate of one rope per hour, but it may not burn uniformly. For example, if you light the rope at one end, it will take exactly 60 minutes before the entire rope has burnt up, but it may be that the first 1/10 of the rope takes 50 minutes to burn and that the remaining 9/10 of the rope takes only 10 minutes to burn. How can you measure a period of exactly 45 minutes? You can choose the starting time. More precisely, given the matches and the rope, you are to say the words “start” and “done” exactly 45 minutes apart.