It’s the Friday Puzzle!

69

cover

Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.

You have two ropes and some matches. Each rope, if lit at its end, will burn for 60 minutes. But the rate of burning is not regular, so cutting a rope in half doesn’t result in a burn time of 30 minutes.

How can you use the ropes to time exactly 45 minutes?

I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.

69 comments on “It’s the Friday Puzzle!

    • Anonymous says:

      Wow, I’m impressed

    • Monty says:

      Easy. You light one rope at both ends, and the other at one end. Wait for the first rope to burn up, then light the other end of the remaining rope.

      The first rope will take 30 minutes to burn, at which point the second rope will have 30 minutes left to go. Lighting it at its other end will reduce this to 15 minutes, for a total of 45 minutes.

    • Police Bureau says:

      Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took.

    • DeepField says:

      I think one of the objectives of Dr. Wiseman with these puzzles is to take a look at the psychopaths that turn up. Almost every week there is one…

    • Dave's Neighbor says:

      My local cycle path is flooded

    • Challenge Monty says:

      Monty – you’ve shown how easy it is to be a twat, but perhaps you’d care to show how clever you really are and post a logical or algorithmic proof of your hand-waving answer.

    • One Eyed Jack says:

      Congratulations, Monty. You have successfully demonstrated what a truly sad and pathetic person you are. All I can do is feel pity for someone so desperate for attention.

    • TarenCapel says:

      Monty, you’re a penis.

  1. robhes says:

    Got it in 45 minutes. I think. Not sure – can’t get used to this new rope

  2. W Ellison says:

    45 seconds

  3. Anonymous says:

    Identical ropes?

  4. The Masked Twit says:

    Took me too long, but not 45 minutes.
    I tried one or two complicated methods, before alighting on the correct solution.

  5. ChrisR says:

    Heard before so knew roughly what solution was, but still had to think for a few seconds.

  6. Steve Jones says:

    A bar has a sign “no ropes served”, into which wanders a gnarled, worn and tangled clump of hemp cord,

    “Are you a rope?” demands the suspicious barman. “No, frayed knot” is the reply.

  7. ctj says:

    i thought that due to bad planning, the ropes were all cut into 3-inch lengths. “simpson’s individual rope-ettes” or something like that.

  8. mgm75 says:

    I have a smart-arse answer but not sure if my thinking is along the lines of the actual answer. I guess I’ll find out Monday

  9. There has to be something wrong with my bleedin’ obvious solution, as it only needs one rope.

  10. Barry Goddard says:

    Is easy with just one rope and one match.

    Split the rope lengthwise and then cut one of the lengths in half. Tie one end of each of the now two short ropes to the same end of the still long rope. You now have a sort of letter Y shape in rope.

    Light the base of the Y.

    When all the rope has burned that is 45mins. And you still have the second original rope and a spare match to repeat the expeiment.

    • Steve Jones says:

      You seem to have assumed that the two lengths of split rope will burn at twice the rate of the original rope, so a complete length takes 30mins and that the two cut lengths each take 15mins, making 45 mins in total. That the two cut lengths each take the same time assumes a constant burn rate, which is explicitly ruled out in the question.

      In short, this is about as wrong as it can get…

    • Dave's Cat says:

      I disagree Steve. Barry’s answer is a work of genius. Well done that man.

    • Barry Goddard says:

      @Stephen Jones

      That you have thought a little and commented is good. Richard likes to encourage thoughtful interactivity.

      When you have thought some more you’ll realize the simplicity of my solution. Or wait until Monday for Richard to endorse it in his own words.

      Thank you for thinking you had a comment.

    • Steve Jones says:

      I know the answer to this particular problem, and your answer bears no resemblance to it, so I’ve no idea what makes you think Richard’s official reply will justify yours. However, I will wait and see. In the meantime, you can explain how your answer works, as (assuming a split rope burns at the same linear rate as an unsplit one), the flame front will take 60 minutes to reach the junction and then each of the shorter lengths will take approximately 30 minutes more (approximately as it’s explicitly stated that if you cut a rope in half it won’t take exactly half the time to burn).

      What this means it will take at least 90 minutes to burn all the rope as you’ve configured it when lit at the base (bottom of the Y).

    • The Lord Manley Fan Club says:

      Steve Jones,
      I wouldn’t put too much store in Richard’s official answer – there may be more than one way of doing this and Richard does sometimes get the answer wrong.
      The problem with Barry’s suggestion is that, as you point out, it makes assumptions as to the burning rates of split ropes. Whatever his assumptions are – and I’m not clear – they are not stated in the puzzle anyway.

      Barry,
      Why don’t you state your assumptions and try to prove Steve and the Fan Club wrong? I for one, won’t play the spoiler card at you.

      TLMFC

    • Ken Haley says:

      What? I don’t get it. The two smaller lengths wll burn much faster than the long piece. So “all the rope” will be burned when the long split rope finishes burning. That’s 30 minutes, if the split rope burns twice as fast as the unsplit rope, or 60 minutes if it burns at the same rate. I don’t see how you get 45 minutes. Regardless, you’re making an assumption about how fast the split rope will burn, and apparently that the two split pieces will burn at the same rate–which is definitely ruled out in the question, as Steve points out..

    • ctj says:

      have any of you worked with actual fuses before? if you take 2 fuses, each of which burns through in 30 seconds by itself, tie them together and light them, it will still take 30 seconds to burn through the combination.

      Steve Jones is precisely right that the “Y” configuration will “take at least 90 minutes to burn.” not just that, but it will almost certainly take more than 90 minutes, and even worse, it is impossible to know exactly how much more, because the rope’s burn rate is not constant.

      there are at least two legitimate solutions to the puzzle that give exactly 45 minutes, but Barry Goddard’s “Y” is not one of them.

    • ctj says:

      i meant to say, “take 2 fuses and braid them together so that their ends touch” or something like that. obviously the situation is different if you put 2 equal-length fuses together into one long fuse that is twice as long.

    • Anonymous says:

      Please let us know what you two solutions are on Monday ctj
      Thanks

    • Dave's Neighbor says:

      But if the rope is half the thickness, The flame will be 50% smaller, so they would cancel each other out

    • Barry Goddard says:

      I see much comment,which I thank you for, but only one way my answer could fail. That would require a very peculiar construction to the rope.

      The rope would have to have a strand that burns very fast (say just a few seconds end to end). That fast burning strand ignites the full length of the rope. The rope then burns to a charred finish in the stipulated one hour. The whole rope burning along it’s full length at once so to speak,

      If that was the ropes construction and my solution desplices the rope into a halve without that special strand then yes my solution will fail.

      Yet then all other solutions which assume a burning from end to end (rather than side to side) also fail.

      If you believe your solution is right then so too must mine.

      Again than you for thoughts, But let us be less dismissive of creative solutions that are not the one you read in a book. The real world allows for truely creative thinking not just memorisation of old tricks.

    • Occam says:

      Would you like to borrow my razor, Bazza?
      I don’t have a spelling guide I am afraid.

  11. Hugh Janua says:

    Err … I have a fully working watch on my wrist. So, what’s the point?

  12. I started with one rope and lit it at both ends. If if is a 60 minute rope then with both ends burning at the same time it will reach 30 minutes when the two burning ends meet. Now I need to take a step back. What I didn’t tell you was that when I lit both ends of the first rope, I also lit the second rope at exactly the same time, but only at one end, let’s call it end “A.” With the first rope burning on its own to calculate 30 minutes for me I know that the 30 minutes will be up on the second rope at the same time. So with 30 minutes determined I only need to burn the second rope for 15 minutes more to come with a total of 45 minutes. The second rope has burned for 30 minutes. When the first rope finished burning (30 minutes) I light the second rope’s other end “B” so that it is now also burning at both ends. The “A” continues burning inward and the “B” end is burning inward. When those two ends meet as they are burning toward one another that will be 15 minutes. Keep in mind the second rope already burned 30 minutes when just the “A” end was lit which leaves 30 minutes left on that rope. Half of 30 is 15, so when ends “A” and “B” are burning the last 30 minutes of the rope and two ends finally meet, it will have been 15 minutes, or half, just as it was done on the first rope. I hope I explained this right. I can draw it better than I can write it. cheryl

  13. snoma says:

    It always surprises me how every single week (without fail) there’s people who post their answers when it states very clearly in the post every single week; “Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took.” The word NOT is even in bold, heavily emphasised.

    Do people just not read the first line or do they not give a shit?

    • Percy Sledge says:

      I used to think it was the same cunt every week, but this week’s has used his full stops correctly, so that dashes that idea (unless he’s been taking classes, I suppose). I think Richard could maybe add a sentence to the top saying that there is bound to be someone spoiling things further down so have a try at the puzzle first.

    • Hugh Janus says:

      Steady on with the language Percy Sledge. My young son regularly looks at the site. Swearing is not necessary

    • Percy Sledge says:

      Actually, Janus, it was you I thought was doing the spoiling, in a variety of poorly-punctuated guises. Your latest offering is of course in keeping.

      1. This isn’t a blog for children, as the subject matter of some of the puzzles should have made clear even to someone like you.
      2. You might need to explain to your son the attempt at scatological wit that cracks us up every week when we read your name.

    • Dave's Neighbor says:

      So Richard’s site carries an 18 rating? Sorry, I did not realise. Hugh Janus’s point (if it indeed be his real name – is he related to Phil McCavity?) is that there is no need for anyone to resort to foul language. Please excercise a bit more self restraint, or alternatively b*gger off to somewhere more appropriate like Pornhub.

    • Trevor Day says:

      Dave’s Neighbor: Thanks for trying but self-restraint doesn’t come into it, I’m afraid. And it stands to reason that puzzles that prompt a series of comments about premature ejaculation are not for the eyes of minors. I’m not familiar with the website you recommend so can’t comment.

    • Anne Elk says:

      Dave’s Neighbor: And if you think this is a suitable forum for children, why on earth would you be referencing sites like the one you mention? Irresponsible to say the least.

    • Anonymous says:

      Trevor Day

      Are you the Trevor Day who posted on 29/1/14 –

      “Yes, it could. Read it again. It is either the narrator or the narrator’s son.”

      without any stated explanation or logic?

      If so, we are all ears to hear your explanation.

  14. Anonymous says:

    No, snoma, I think they just want everyone else to realise how clever they are. Not.

  15. Anonymous says:

    No snoma, I think they just want everyone else to realise how clever they are – not!

  16. Nick says:

    The key is how should you interpret, “But the rate of burning is not regular…”. It’s possible for a rope to burn in such an irregular manner that there is no solution. But is that irregular way of burning a reasonable interpretation of the above?

    • Ken Haley says:

      Knowing the answer (which has unfortunately been posted by others), can you give me an example of how it’s possible for “a rope to burn in such an irregular manner that there is no solution.”? Remember that we’re referring to the “rate of burning” and not something weird, like… part-way through another section of rope spontaneously catches fire and starts burning both ways from there, or… the outside burns much faster than the inside, or… whatever. The rope’s RATE of burn is what is irregular, nothing else.

    • Nick says:

      There’s no solution if sections of the rope can burn faster in one direction than the other. That’s quite irregular, but refers only to “rate of burning”.

    • Andy R says:

      Good point, Nick. We need to assume that the rate of burn at any point on the rope is independent of the direction in which it is burning. Otherwise we’re up that well-known creek with less than one paddle.

    • Ken Haley says:

      I have to concede the point of that the rate of burn being different depending on direction would render no solution; however, I can’t imagine how that would be possible with real ropes.

  17. Hula Moo says:

    Couldn’t get the ropes to light because the matches and the rope were damp.
    Bloody weather.
    Used the clock on my phone instead.

  18. Dope-on-a-Rope says:

    Is Richard’s new book “Night School” the place you go to learn to be a knight?

  19. ubi dubium says:

    Took about 2 minutes to get it. I had not heard this one before, so good one this week!

  20. Louise says:

    Three minutes

  21. John says:

    30 seconds

  22. Julia says:

    had to think about it for a minute or two, even though i was already familiar with the puzzle.

  23. h3rne777 says:

    Dear Fellow Rats. After following this blog for some time, it occurs to me that the predictable appearance of “spoiler” posts, profanity or indeed any other of the nuclei about which the weekly flame “skirmishes” form, could easily be Richard himself. I quite like the idea of being in the test group of some online-community-focussed research project. I fully expect we shall be the subject of a chapter of Richard’s next book…

  24. toiyeuteopro says:

    Chồng nói với vợ: “Em thường mang bức hình của anh theo khi đi làm. Vì sao vậy?” Vợ: Khi nào có vấn đề, dù có khó khăn đến thế nào, em nhìn vào bức hình của anh và vấn đề đó liền biến mất.
    http://wapnam.com

    Chồng: Em thấy chưa, em thấy anh tuyệt vời với em như thế nào chưa? Vợ: Vâng, em nhìn vào bức hình và tự nhủ: “Còn vấn đề nào to hơn vấn đề này chứ?”.

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