Two Ropes
February 14th, 2007 at 3:32 am (riddle)
You are given two ropes and a lighter. This is the only equipment you can use. You are told that each of the two ropes has the following property: if you light one end of the rope, it will take exactly one hour to burn all the way to the other end. But it doesn’t have to burn at a uniform rate. In other words, half the rope may burn in the first five minutes, and then the other half would take 55 minutes.The rate at which the two ropes burn is not necessarily the same, so the second rope will also take an hour to burn from one end to the other, but may do it at some varying rate, which is not necessarily the same as the one for the first rope. Now you are asked to measure a period of 45 minutes.How will you do it?
El Smarhe said,
February 14, 2007 at 5:17 pm
Light rope 1 at one end. At the same time light rope 2 at both ends. When rope 2 is done burning which would take a total of 30 minutes, light the other end of rope 1 which has been burning from one end for 30 minutes. After you light rope 1’s other end it will take 15 more minutes for it to completely burn, leaving you with two completely burnt ropes 45 minutes later.
Ali said,
February 14, 2007 at 6:41 pm
light 1 rope from both ends. this should take 30 minutes to burn. When this finishes, take the second rope and fold it in half and light both “ends” (really four places). This one should then take 15 mintues to burn, giving you 45 mintues.
El Smarhe said,
February 17, 2007 at 9:06 pm
But Ali, what if one half of rope 2, the one you folded in half, takes 50 minutes to burn while the other half takes only 10? Then one half would burn up after 5 minutes with the other taking 25. We don’t know the rate of burn for segments of rope.
isis merril said,
February 19, 2007 at 3:48 pm
Can’t u just tie a knot and light one end and then the whole thing will catch on fire? Or is that not allowed?
George said,
February 22, 2007 at 6:51 am
Light one rope at both ends. This should burn in 30 minutes. While this is burning. Fold the second rope into four folds. When rope one finishes burning, light the second rope at the four folds. This should burn in 15 minutes for a total of 45 minutes
kenny said,
February 26, 2007 at 7:18 am
step 1 Measure both ropes.
Step 2 Light both ropes at one end,
Step 3 time 45 minutes. and cut the rope.
Step 4 measure the remaing rope and deduct fro the original lenght for rope 1 , repeate for rope 2.
The answer is the Answer, the question does not tell us which rope to burn it mearly asks for a method..
anonymous said,
March 5, 2007 at 7:56 pm
with a watch or with a clock of some sort.
PGM said,
March 6, 2007 at 3:26 pm
Presuming that we have to use the ropes to measure out 45 mins, you cant fold the rope in 4 because there is no uniform burning along it. Hence 1/4 may burn in 1 min, the next in 10 min, the next in 20 min the fianl bit in 29. This is the same idea for burning it at both ends 0.5 may burn in 1 min the other takes it time! As long as the ropes have non-unifrom burning rate I dont think its possible
PGM said,
March 7, 2007 at 2:29 am
Two new thoughts-
1. Place the two lengths side by side. Move the one on the right through 270 degrees. The idea is that the 2 ropes are synonomous with the hands on a clock hence moving one through 270 degrees is equal to 45 minutes on a clock face.
2. A minute is 1/60 of a degree. Hence 45 mins is 0.75 degree. If you can manipulate the ropes so that they are at 0.75 degree to each other you will have 45 mins
Anonymous said,
March 9, 2007 at 9:31 am
Actually the riddle asks how you will measure a period of 45 minutes. All of this other information is erroneous. I stick by my answer of clock or watch, or a timepiece of some sort.
PGM said,
March 12, 2007 at 11:57 am
The answer is twenty-three. Its always TWENTY THREE. Three divided by two is .666 the number of the devil. 48 divided by 8 + 17=…23. Surprised? don’t be.
Thirumalai said,
April 11, 2007 at 9:53 pm
Why do most of the people here assume that the whole rope will burn
in 30 mins if lighted simultaneously from both the ends ??
The riddle says that the burn rate is not uniform and not to assume that
half the rope will be burnt in 30 mins. So the same reasoning and logic
should apply when lighted from both the ends. Please correct me if you
think that I am wrong anywhere ?
Krenn said,
May 15, 2007 at 11:16 am
Thirumalai – it doesn’t matter that the burn rate isn’t uniform *provided* you don’t care where the flames meet.
Think of it like this: Suppose you let the rope burn for half an hour. Call the point it reaches X. After a half hour, we know it will take another half hour to burn the rest of the rope, making it an hour total. So if you lit from the other end, it would take half an hour to burn that end up to point X.
That means that when you light the rope at both ends, both ends burn to point X – wherever that is on the rope.
buey said,
May 15, 2007 at 1:33 pm
The parameters are incorrect, as they prevent a solution. The target time should be 1.5 hours, not 45 minutes. The only times that the ropes can measure are 1 hour (burning one end) and 1/2 hour (burning both ends simultaneously). There are no other measurable increments.
Burning one at one end, then the other at both ends (or vice versa) yields the least obvious predictable result (1.5 hours vs 1 or 2).
buey said,
May 15, 2007 at 1:37 pm
Disregard my last response, it was wrong. Here is the correct solution:
1 – Light one rope at both ends, and light the other at one end.
2 – When the first rope stops burning, light the other end of the remaining rope.
When the last rope stops burning, 45 minutes have passed.
erikpurne said,
May 15, 2007 at 4:51 pm
What is wrong with you people? The very first post is obviously the right answer. As for the rest, it seems clear that some of you are joking (best answer: tie between ‘clock’ and ’23’), some others I can merely hope…
Chris said,
May 15, 2007 at 10:45 pm
Knowing that I can digest one piece of rope in 22.5 minutes, I would simply eat both pieces of rope and wait until they are digested.
Holly said,
July 7, 2007 at 1:34 am
No other equipment can be used. I’m too lazy to use the method described in the first post. Now what?
Start counting seconds.
Nikola said,
April 11, 2008 at 1:45 am
Another solution: light one rope from both ends and let it burn for 30 minutes; then light another rope from both ends and in the middle and let it burn for 15 minutes.
Justin said,
July 15, 2008 at 2:49 pm
Everybody is wrong.
1. Cut one rope in half
2. light one of the halves on both ends
3. At the same time light on end of the whole rope
4. The halve will burn out in 15 minutes
5. Put out the flame on the whole rope when the half rope is finished burning
6. You are now left with exactly 45 mins of rope
Justin said,
July 15, 2008 at 2:51 pm
Correction: burn both halves at both ends
Travis said,
August 24, 2008 at 7:01 am
Justin learn to read it clearly says “two ropes, lighter, no other equipment” unless you plan on gnawing the rope theres no way you can CUT it…
Rachael said,
March 13, 2009 at 6:10 am
cut it with the lighter by burning it in the middle.
Iniffurseni said,
March 20, 2009 at 5:59 am
very intresting
Patrick said,
September 2, 2009 at 10:04 am
I’m pretty sure the first solution is the only way to do it
nitij said,
October 7, 2009 at 9:21 am
Bullshit..all confused piece of meat……none of these are answers…for sure …all confusing and doesnt worth even to pay brain in !!!
davidl said,
October 21, 2010 at 6:21 am
Light both ends on one rope, and only one end on rope two (at the same time). when rope with both ends lighted burns up, light last end of last rope. 45mins.
come on........ said,
November 18, 2010 at 9:36 am
DAVIDL wins… every other answer posted before is WRONG
Rajesh Kumar said,
September 19, 2011 at 11:33 pm
Agree with Davidl’s solution.
Kris said,
November 24, 2011 at 3:41 am
And how’s it different from the very first post by “El Smarhe” ?!
Jacky said,
July 3, 2012 at 8:03 pm
Light both ends of rope A and one end of rope B After 30 minutes, rope A will be completely burned up and there would be 30 minutes of rope B left. Light the other end of rope B; it will burn up 15 minutes.
Emma said,
July 3, 2013 at 6:55 pm
I read through all the comments to finally realize that El Smarhe’s comment, the very 1st one, was correct. bahaha
Vineet said,
November 17, 2013 at 3:16 pm
El Samarhe u r a true genius in solving this problem unless u copied it from somewhere else!
But you deserve credit no matter what by solving the problem and providing the most logical series of explanation…rest of all is crap and hypBULLOthetical
Vineet the ideeeet said,
December 30, 2013 at 3:39 pm
Learn to spell.