It’s the Friday Puzzle!

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An old classic this week.  You have a straight-sided glass of orange juice that is filled to the brim like this…..

…and would like to share it with a friend.  You have no other drinking vessel, and nothing with which to measure the depth or content of the glass.  How do you make sure that you both get a fair share?

As ever, please do NOT post your answers, but do say if you think you have solved the puzzle and how long it took.  Solution on Monday.

And while you are thinking about it, here is a terrifying video (via@rpaulwilson)

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.

76 comments on “It’s the Friday Puzzle!

  1. Anonymous says:

    I think I did…3 seconds…

  2. Miche says:

    Dead easy. But probably not because I worked it out for myself: more likely I was taught the answer when young.

  3. Anonymous says:

    You don’t, because that is socialism.
    /s

  4. Katharine says:

    Yay, at last I can be one of smug ones – I got it instantly

  5. Mertens says:

    Fair share or equal share?

  6. Morgan says:

    Define fair.

  7. An answer was immediately obvious to me, but I stopped to think once I realized it’s a cylinder in 3D. But, the answer is the same.

    • Alocaly says:

      I had exactly the same reflexion :
      * It’s obvious
      * Damned, wait : it’s in 3D, and it’s a cylinder !!
      * Ahah, i found the trap…
      *…
      * Hum ??? It doesn’t change anything : it’s still the same obvious solution….

      And while I’m writing on this post, I’m not a real fan of the video…
      I don’t like such a simple brutal scene exposed…

  8. haven’t everyone had to solve this irl at some point?🙂

  9. Boy, you have some smart people here… I think I have it. I got it yesterday when you were thinking about it.

  10. Andrew says:

    Took about 3 seconds but I can’t remember whether I’ve seen it before.

  11. David Hanley says:

    Assuming fair means equal, just a second or two

  12. Hugh Jarse says:

    I really do not see the point of posting “I’ve done it” comments. What a bunch of sad, lonely people

    • I’m with you on this one, Hugh, but Richard does ask people to post how long it took. Never really worked out why…

    • z says:

      because it says in the post “As ever, please do NOT post your answers, but do say if you think you have solved the puzzle and how long it took. “?

    • jramills says:

      I always figured he was either greasing his ego, or using the stats to gain advertising revenue….

    • jtradke says:

      Why does this happen in every Friday Puzzle? Richard specifically tells people to post how long it took them to answer, and then some schmuck insults everybody for doing exactly that.

      Don’t feel bad, Hugh, I didn’t figure it out yet either. Doesn’t mean I have to berate others to compensate.

    • Bart says:

      Presuming that this is a troll, and huge arse is having some fun, but I’ll play along.

      ‘I really do not see the point of posting “I’ve done it” comments. What a bunch of sad, lonely people’

      Are they worse than people who have no reading comprehension skills?

  13. Tom Green says:

    Does this have anything to do with a crow?

  14. Hugh Jarse says:

    …. Sorry forgot to add the word “smug”

  15. Kanzie says:

    You get a stick and beat your friend on the head until he/she says it’s fair?

    (I did get a more reasonable solution a couple of seconds after this solution came into my head…)

  16. King Conan says:

    Whomever is the strongest will kill the other and drink the whole thing.

  17. Lucy Lastick says:

    Hugh, I have not solved it yet. I thought that I Ought to tell everybody

  18. leslielu ♔ says:

    Pretty sure I got the right answer within seconds.

  19. ... says:

    Pour it all away. That way you both get nothing, so it’s fair.

  20. Anonymous says:

    As long as rock-paper-scissors is involved, the outcome is fair AFAIC.😉 Actually, because I had to work this one out IRL once for a can of railroad spikes, I had the answer lodged in a deep memory crevasse and was able to retrieve it with a minimum of effort. But I do remember that it was an “ah ha!” moment when I figured it out the first time.

  21. duxallinarow says:

    As long as rock-paper-scissors is involved, the outcome is fair AFAIC.😉 Actually, because I had to work this one out IRL once for a can of railroad spikes, I had the answer lodged in a deep memory crevasse and was able to retrieve it with a minimum of effort. But I do remember that it was an “ah ha!” moment when I figured it out the first time.

  22. Chris Emerson says:

    Solved it, but Matt Parker asked on Twitter… What if I have n friends?

  23. vitamentis says:

    Got it. Took a few minutes. Still, I wouldn’t literally try my own solution.personal preference.

  24. Ophelia Balls says:

    I’ve not solved it either

  25. Got it! I never get these. Can’t believe I worked this one out (took about half a minute).

  26. Berhard says:

    easy…instantaneously…

    but it is alos possible to use a refrigerator.. .

  27. stuart hamilton says:

    Got it instantly. Might have been a bit more tricky if the picture were a photo

  28. Shane McKee says:

    Hey, Matt Parker: n straws, 3,2,1, GO!🙂

  29. Shane McKee says:

    Sorry – n+1 (need to let Matt have a drink too…)

  30. Vik says:

    Oh! About 3 seconds for this one!

  31. Bob Anderson says:

    I occasionally use this to keep half a can of baked beans for a future meal! (Assuming that’s the solution you are referring too of course).

  32. M says:

    3 seconds or so.

  33. Bletherskite says:

    I looked at the questions and though urgh and then looked at the pic and answer jumped out so about 5 secs for me.

  34. mittfh says:

    There’s one pretty obvious solution that doesn’t involve any additional equipment whatsoever – the only proviso being that both you and your friend have similar sucking capacity.

    However, it’s probably not what Richard’s looking for.

  35. Phil McCavity says:

    If it nae got alcohol, I ain’t drinking it pal

  36. yes it’s easy!
    i knew it as soon as i read it. …
    you come across this almost everytime you drink from a glass.
    but to be absolutely sure i also checked it mathematicaly by calculating an example, that took me a few minutes to write it down

  37. kag says:

    About 10s

  38. Phil McCavity says:

    Got it 2 minutes BEFORE it was posted

  39. TS says:

    Laurel & Hardy faced a similar problem once. Jump to 4:30 if your attention span is up to classic slow burning comedy.

  40. Noel says:

    First time in a long time I think I worked it out. About 5 mins.

  41. Rob says:

    About 2 seconds, as long as you’re not too bothered about hygiene…

  42. Tsuyoiko says:

    I think the answer is not to be such big babies and worry about one person getting a few millilitres more; the first person should just drink what looks like half and the other should finish it off. But I’m going first😉

  43. Chip says:

    I think to make it more realistic, the problem should say “sibling” instead of “friend”. Friends would just estimate & be done with it. SIBLINGS, however, require ultimate precision. But then, of course, a method to make sure they don’t have to drink from the same cup would be needed, because, y’know… cooties.;-)

    I’ve come across this before, so my solving time doesn’t really count.

  44. As with others, got it nearly instantly.

  45. Jeremy says:

    Well done, Todio, you gave it away.

  46. Anonymous says:

    The obvious solution requires one share to be consumed before the other comences so I’d abandon that in favour of just passing it back and forth one glug at a time.

  47. If you want to be fair then it all belongs to the government,if you want equal a few seconds

  48. Rob says:

    Visually obvious at once. Thanks for easy puzzle.

  49. Took me about ten minutes. But I was distracted by that horrifying video. Eek!

  50. Danny says:

    Got it. Took me a while. I’d say around 5 minutes.

    • Danny says:

      Though ANY amount I share with my friend is “fair,” including not sharing any at all. That’s MY orange juice.

  51. Tanya says:

    Neither of you drink it and give it to some homeless guy.

  52. Mark says:

    My word, everybody seems a bit cranky today. I know I am, because (a) I’m not even supposed to be drinking OJ, and (b) that video is damned upsetting.

    I know I must have seen this puzzle before. It seemed trivially easy, and I’m not that clever.

  53. hcubic says:

    very easy

  54. Confmat says:

    Easy!

    You cut the glass in half then let your friend choose which half they want.

  55. Dee says:

    ~7 seconds

  56. Just a few seconds; nice and easy this week.

  57. Kelly says:

    Re: Puzzle

    Nice! I’ll pass it on to friends…

    Re: Crash

    Yikes! I’ll pass it on to friends…

  58. duckfish says:

    20 second.I didn’t managed to figure out the answer at the first in instant, but when I saw saw a comment”3 second” on the top, I suddenly know the solution, pretty obvious,

    I’m slow :(
    seemed like most people got it within 10 seconds

  59. Benny says:

    Negative 3 minutes

  60. Vince says:

    I couldn’t figure it out yesterday, so I came back to it today, and got the answer almost instantaneously. Then I thought it might be different in 3d, but then I figured the answer is the same

  61. erica says:

    If it is fairness that is the issue (ie all parties can agree that the share they get is a fair one) then a simple solution extends to any number of sharers

  62. moo says:

    Haven’t the foggiest.

  63. Bart says:

    Presuming they go sequentially, the question becomes, after the first person takes their “equal” share, is the glass then half full or half empty?

  64. I think I got this one right away. Not a whole lot to think about really unless, of course, you bring into account whether it is fresh squeezed or pulp free. Pulp does cling to the side of the glass and who can say how much?

  65. rodney says:

    this one was easy i got it instantly

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