The Burning Rope Problem

Author: Kasper B. Graversen, 23/07/08
Keywords: IQ Test, Logic test, Fun
Abstract: A mindbending puzzle that'll keep your brain going for hours! Given two pieces of rope, and some fire identify when 45 minutes has passed.
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The Burning Rope Problem

I really like this puzzle! Primarily because it is so abstract and seemingly missing essential information to solve. You'll be puzzling for hours!

You are equipped with two ropes and some matches.

Your goal is by burning the rope(s), to identify when exactly 45 minutes has passed. All you know about the ropes is:
  • The ropes are of different length and you don't know how long they are.
  • Each rope burns in precisely 1 hour.
  • Each rope has a non-uniform density, meaning it is thicker at some points than others. Consequently, burning half a rope cannot be guaranteed to take 30 minutes.
Simple, eh? ;-) You may feel as though you need more information. Worry not, once you figured out how to get started, you are almost at the finishing line!







Hint 1

A rope has two ends







Hint 2

Setting fire to both ends of a rope simultaneously burns the rope twice as fast, hence it's burning time is 30 minutes.







Solution

  • First lit rope 1 in both ends and one end of rope 2.
  • After 30 minutes rope 1 is completely burnt and rope 2 still has 30 minutes worth of burning time in it.
  • Lit the other end of rope 2. This will burn the remaining rope in 15 minutes, totaling the burning time to 45 minutes.

Have fun...



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