Posted on 05/15/2025 9:41:05 PM PDT by Red Badger
This simple puzzle recently appeared in a members-only newsletter. Think you can crack it?
You have two ropes coated in oil to help them burn. Each rope will take exactly one hour to burn all the way through. However, the ropes don’t burn at constant rates; there are spots where they burn a bit faster and spots where they burn a bit slower, but it always takes one hour to finish the job. With a lighter to ignite the ropes, how can you measure exactly 45 minutes?
https://www.popularmechanics.com/science/math/a24210/solution-to-riddle-of-the-week-6/
I look at my watch?
Just the burning ropes......................
😎..........................
Burn one rope at both ends, and light the second rope at one end. The first rope will burn through in a half hour and the second rope will burn halfway through. Light the remaining second rope at both ends and it will take 15 minutes to burn through.
Or just look at your watch or your phone.
I didn’t read the solution, but that kind of a brain teaser sounds like something a few fictitious famous men may have tried.
Sherlock Holmes
Sir Lawrence of Arabia
Macguyver (sp)
Clutch Cargo (OK, Boomer!)
Light both ends on the first rope. That will be 30 minutes. Fold the second rope burn the middle then light up the other ends . That will give 15 minutes.
To measure exactly 45 minutes using two ropes that each take one hour to burn completely, despite non-uniform burning rates, follow these steps:
Thus, you can measure exactly 45 minutes by timing from the initial lighting until the second rope is fully burned.
Tie both ropes together, light them on fire, and when they are 3/4 burned....
45 minutes.
(I did not look at the answer first. FWIW)
Throw both ropes on the barbie.
There are a lot of missing variables. BUT. Not knowing what they mean by taking “one hour to burn all the way through “ I surmise they mean lighting one end and burning like a wick. If that’s the case. Fold one robe in half and light both ends for 30 minute burn. Fold the other rope two time in half. When the 30 minute rope is finished burning light the ends on the other folded rope for a 15 minute burn.
I’ll take “with a watch”…… for $400 Alex!
All right. This is crap. For one thing, you have to pay for the answer.
I see Larry posted the answer.
But it’s not an answer to the question as asked and illustrated, nor is it right in terms of the question as the answer purports it to have been asked. The illustration and wording indicates the rope is being lit at the side, not the end, and burning through the rope.
Here’s their answer without the solution.
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“Because the ropes burn at inconsistent rates, you can’t simply measure 75 percent of the way down one rope and call that 45 minutes. The rope might burn slightly faster or slower in that last 25 percent. However, if you light one of the ropes on fire at both ends at the same time, it will burn up in 30 minutes, even if one side burns faster than the other.
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Larry’s answer makes sense after ignoring a mistake “ the second rope will burn halfway through”. The question explicitly said the rate of burning does not correspond to length of rope burn, ie the rate varies. The time will be halfway through but the rope could be any percentage burned up.
There was a good one on Columbo.
You are 100% right on how poorly the question is stated and presented.
Ultimately it makes sense. But after making assumptions and ignoring the rhetoric “exactly 45 minutes”. No way it could be exact in the rope and one lighter scenario.
Nope. Different burn rates throughout the rope means no length of either rope can be said to provide any measure of time.
Even folding one rope does not mean the two halves of the rope finish burning at the same time. One half could burn in 5 minutes and the other in 55 minutes.
It is a puzzle with a wrong answer.
I didn’t check, but how about light them after they’ve burned for 15 minutes.
The example is full of problems but the math problem is ok.
Two things.
Both will disintegrate in the same amount of time after being touched in one spot. Make it one hour.
They will disintegrate at twice the rate if touched in two spots.
So touch one at two spots, other at one.
First is gone in 30 minutes.
Second is halfway, has 30 minutes to go.
Touch that one again to double the rate. It will be gone in 15 more minutes. Rather than 30.
30 minutes plus 15 for it to be gone.
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