Brain Teaser – 2 Ropes – Answer
First, I apologize for being so late on this. Second, I was surprised by the number of people who got the answer right! I have listed the first answer I got back along with an alternate answer to the question below.
Question: http://www.twentysomethingsense.com/2009/04/brain-teaser-2-ropes.html
You are given 2 pieces of rope that are exactly 6 feet long each and a box of matches. Each rope is flammable and takes exactly 1 hour to burn. Each rope burns at a different and inconsistent rate (ie: it might take 10 seconds for the first 5 feet of rope to burn and the remaining 59 minutes and 50 seconds for the last foot of rope to burn).
How do you know when exactly 45 minutes has passed?
Answer:
From James Dixon – First to respond!! – You are given 2 pieces of rope that are exactly 6 feet long each and a box of matches. Each rope is flammable and takes exactly 1 hour to burn. Each rope burns at a different and inconsistent rate (ie: it might take 10 seconds for the first 5 feet of rope to burn and the remaining 59 minutes and 50 seconds for the last foot of rope to burn). What are these, Harry Potter ropes?
From Ben Beyerlein – Alternate Answer – Take Rope 1, fold it in half and light both ends. For Rope 2, only light one end. When Rope 1 is done you know its been 15 minutes, therefore the rest of the length of Rope 2 takes 45 minutes.
**Alternate Answer corrected by Rick – “That’s not quite right. Because the rope doesn’t burn at an even interval there’s no guarantee that you can make first rope burn in 15 minutes. (Imagine a certain 1 inch segment takes 40 minutes. The half containing that segment will take at least 20 minutes to burn, burning at both ends.)
Instead, simultaneously light both ends of one rope, and one end of the second rope. When the rope lit at both ends burns out (after 30 minutes), light the second end of the second rope, which now also has 30 minutes worth of burning left to do. Lighting that second end means the 30-minutes worth burns out in 15 minutes instead, totaling 45 minutes.”
Thanks for all of the participation! We’ll be working on adding more puzzles over time.
“Alternate Answer – Take Rope 1, fold it in half and light both ends. For Rope 2, only light one end. When Rope 1 is done you know its been 15 minutes, therefore the rest of the length of Rope 2 takes 45 minutes.”
That’s not quite right. Because the rope doesn’t burn at an even interval there’s no guarantee that you can make first rope burn in 15 minutes. (Imagine a certain 1 inch segment takes 40 minutes. The half containing that segment will take at least 20 minutes to burn, burning at both ends.)
Instead, simultaneously light both ends of one rope, and one end of the second rope. When the rope lit at both ends burns out (after 30 minutes), light the second end of the second rope, which now also has 30 minutes worth of burning left to do. Lighting that second end means the 30-minutes worth burns out in 15 minutes instead, totaling 45 minutes.
Will, this is what I also sent you:
Light both ends of 1 rope, and simultaneously light only one end of the other rope. When the first rope finishes burning (30 mins.), light the other end of the second rope. Because the second rope was half burned in time, lighting the second end of the second rope will cause it to burn twice as fast, or in another 15 minutes. 30+15=45.
Rick – Good catch!! I was wondering why I had never heard that answer before … Thank you for the correction.