It’s the Friday Puzzle!


On Saturday I am interviewing the immensely talented Derren Brown as part of the Edinburgh International Science Festival.  The event sold out very quickly and I am looking forward to it.  If you were lucky enough to get a ticket please note that there has been a venue change, and that the event will now take place in the Church Hill Theatre.

So, to the puzzle…..

Imagine that you are standing on the banks of a straight river.  Across from you is a sign on the opposite bank.  To your right is a tree.  You have nothing to measure with and cannot cross the river.  How can you figure out roughly how far you are from the sign?

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.

Oh, and feel free to post any questions that you would like me to ask Derren.



83 comments on “It’s the Friday Puzzle!

  1. CH23 says:

    there’s 2 possible ways, if it is what i think it is.
    Took me 6 seconds.

  2. Depends if the sign says how wide the river is.

  3. Navneeth says:

    Take out my smartphone and look up the river’s width on Wikipedia.

  4. Gib says:

    I figured a good way out after about a minute.

  5. Opening question “So, Derren, how do you do all your tricks”.

    You could have cards with different anticipated responses dotted about your person:
    • Really rather well, thankyou
    • I could tell you, but I’d have to kill you
    • with real magic, there is no trick
    • Mostly magnets and mirrors
    You then produce the one closest to his answer, or if you ‘miss’, just move on.

    • Ian says:

      You’d need a card that says “magic, suggestion, psychology, misdirection and showmanship”. Obviously.

  6. follystone says:

    I have a way that will work, but it does rely on the availability of a commonly found item not mentioned in the puzzle

  7. The Pedant says:

    If you have “nothing to measure with” – no imagination, powers of deduction etc – it is axiomatic that you can not solve this problem.

    Took me about a minute to get a rough answer.

    • Lazy T says:

      I’m also wondering which of my body parts I am without.

    • waddett says:

      Good thought Lazy T. If I was blind don’t think I could do this problem, but presuming I got to the river bank I presume I can also get to the tree.

  8. dharmaruci says:

    this is very difficult perhaps unsolvable. my thought is this, i can move along the bank until tree and sign appear same distance away. then i have no way of measuring distance to tree according to ricard’s question.

    if i could measure that (which i would call A) then all i need to do is measure distance from spot A to distance were sign is perpendicular across river (is spot B). then A minus B is the anser.

    but without ability to measure my side of river i must await monday as my only solution.

  9. cjquines says:

    I got it! After leaving the puzzle alone for a while.

  10. There is a saying isn’t there “a man is his own yardstick” I can think of quite a few ways of accomplishing this. That river is remarkably straight isn’t it.

  11. edgar1975 says:

    I quickly found a way to measure it, but it wouldn’t be so quickly to measure it if I were at the river.

  12. MoanyOldGoat says:

    I think that’s very easy, in fact that easy Im doubting if Im actually right!

  13. Daft Punk says:

    Two solutions

    1. Look it up on Google Earth and measure the gap on your PC screen and use the distance scale and calculate.

    2. Wait for a passer by on the other side, throw one end of a rope to him and ask him to hold the end to the sign. Pull your end to tighten the slack and mark the rope where it meets you. Retrieve rope and measure the length from the end to the marked point.

    I await a reply fron the pedants among you …..

  14. Something SohCahToa based with pacing to the tree and estimating angles. I can’t be bothered doing the trig :-\

    • Steve Jones says:

      There is a rather simpler way, which the Greeks would have worked out.

    • Michael Sternberg says:

      Euclid will do, and without climbing the tree. – It’s a matter of congruence and some legwork – if the river bank is reasonably flat.

      Took me a minute to translate the question to reasonable conditions.

  15. Nick Day says:

    Hmm, you could ask him whether he’d rather be a dental receptionist or a mental deceptionist?

  16. Mad Kev says:

    Just look across and roughly work it out. So what’s the puzzle?

  17. mittfh says:

    I have a couple of answers, one of which involves trigonometry and one assumes the sun’s shining…

  18. Dave says:

    I’d like you to ask Mr. Brown whether he thinks he’s doing the public understanding of science a disservice by duping people into believing that his tricks are due to his amazing ability to read body language and psychology when in fact they are simply clever conjuring tricks.

    • Dave says:


    • Dave says:

      To clarify, that wasn’t Dave seconding his own post, I’m also a Dave.

    • Anonymous says:

      Why would he care?

    • Dave says:

      This is the first Dave answering Anonymous – I don’t know, that’s the point of asking him whether he does or not.

    • Dave says:

      I’m Dave and so’s my wife

    • Steve Jones says:

      what is this, a cheap remake of the climactic line from Spartacus?

    • jramills says:

      More Life Of Brian than Spartacus.

    • Gus Snarp says:

      Yeah, that’s sort of my question too. The OP, not anything about classic movies.

    • One Eyed Jack says:

      Derrin has always stated that he is a “Mentalist” and possesses no special powers. It’s psychology combined with guiding and misdirection.

      He’s an entertainer, and more forthright than the charlatans that put themselves forward as “psychic.”

      What is the problem?

    • Dave says:

      “Derrin has always stated that he is a “Mentalist” and possesses no special powers.”

      Except that he’s been quoted as claiming that some of his results are achieved by reading body language and psychology, when every psychologist will tell you this is very likely to be bollocks. So, it is acceptable when Derren Brown misleads the public in this manner, but when Uri Geller misleads the public by claiming that his results are due to psychic powers, it is unacceptable. Interesting.

    • One Eyed Jack says:

      There is a monumental difference between saying you “read body language” and saying you are psychic. Nothing supernatural is claimed.

      This is a non-issue when compared to psychics, astrologers, fortune tellers, mystics, faith peddlers, and the rest of their ilk.

    • Dave says:

      “There is a monumental difference between saying you read body language and saying you are psychic.”

      Well, in this case they are both be lies (neither are of the ‘honest’ variety). I’m curious as to why one lie is acceptable and the other is not.

    • Match says:

      @One Eyed Jack

      Not really when it is impossible to acheive his effects through psychology. If his actual methods were the methods he claims to use, and the methods that the average person believes he uses, it would in fact be supernatural. It’s like if Uri Geller went around denouncing psychics and claiming that he can bend spoons by exploiting quantum mechanics or something.

  19. SofARMaths says:

    Involves a bit of walking?

  20. ola says:

    i will walk to the tree and measure how it long by my leg then calculated the distance to the sign by knowing the angle equal to 90 degree so you will now the left side.

  21. AMWhy says:

    If you can get an exact answer I’d love to see it. I have an approximate answer but it could take a while to work out – quite a bit of walking involved. Nothing fancy needed though, or at least, nothing I don’t always have with me.

  22. MartianMan says:

    Got it in about a minute – elementary really if you are only looking for a rough answer. Enjoy the puzzles!

  23. Mick says:

    Got it in a minute

  24. Tom (iow) says:

    How close is ‘roughly’? I have various trigonometric ideas but I’m not sure any is more accurate than ‘estimate the distance to the sign using my judgment’.

  25. Erik R. says:

    Cut down the tree and make a raft. Then you take over a cannibal first, then come back for the first missionary.

  26. Anonymous says:

    Finally got around to thinking about it, and about 1 minute to then solve it. No maths involved, especially not trig!

  27. Got it so you can know the distance almost exactly (but you still don’t have anything to measure it with but that doesn’t matter as long as you know the distance right?) and I didn’t even need to swim.

  28. Ian says:

    Oh, that’s a clever one, thanks 🙂 There is an obvious answer that came to me instantly; and, of course, there are mathematical solutions, so either way, it’s a good one, thanks 🙂 Hope all goes well with interviewing Derren Brown. Is there anyone, by the way, whom he admires as being immensely talented or any kind of inspiration for his own work and lifestyle, with his clever techniques and his excellent art? Just wondered if there are any people, influences or events that have inspired that quick and clever, eclectic mind of his 🙂

  29. Gus Snarp says:

    I’d ask Derren about how he blurs the line between being a skeptic and a mentalist. There are the Richard Wiseman, James Randi style skeptic magicians, the “honest liars”, who seem to draw a hard line between when they’re debunking and when they’re performing, who won’t tell how the trick is done, but make it clear when it’s a trick and when they’re doing something real. Then of course there are the Uri Gellers of the world who claim it’s all real, but Brown seems to me to represent something in the middle. He debunks faith healers, but he does these hypnosis shows and these human lie detector bits where it’s hard to see how much is real suggestion, real ability, and how much is a trick. It troubles me a bit, to be honest, so I’m always wondering what’s real and what’s not, and not in a pleasant way. Maybe I’m the only one, but that’s the topic I’d discuss with him. After I found a way to explain it better than this.

    • Steve Jones says:

      The one thing Derren Brown never claims is that he achieves his results through supernatural means. However, as an illusionist, he does keep to himself the right to misdirect. For the most part, none of his psychology based explanations can be taken completely at face value, and in some case, he uses the classic tricks of illusionists. Of course there are some exceptions. In “The System”, he did indeed show how they used confirmation bias to delude one individual into thinking she was a guaranteed winner. However, even then there was an apparently impossible twist at the end.

      Ultimately, Derren Brown is a showman, and wonderful at misleading both his audience and the subjects of his programmes.

    • Dave says:

      “Ultimately, Derren Brown is a showman, and wonderful at misleading both his audience and the subjects of his programmes.”

      You could say the same about Uri Geller

    • Steve Jones says:

      “You could say the same about Uri Geller”

      The difference is that Uri Geller claims he is achieving his results through supernatural means. He has been very vocal in denying that he is just an illusionist.

      nb. is this yet another of the many Daves?

  30. Eddie says:

    Walk to the source of the river and then back to the point on the other side, adding all the discrete vector distances as you go. This will give you a vector distance across the river from which you can take the scalar answer.

  31. Vero! says:

    Is like playing hide and seek! (6s)

  32. Ken Haley says:

    Got it in about 15 minutes. Solution needs no math–just walking and counting steps.

  33. Sara says:

    I am not dave. Nor spartacus. I have resorted to asking my daughters to solve the problem. I am a brain dead maths teacher.

  34. Lazy T says:

    Ask DB if he knows
    what goes up the chimney down but not down the chimney up?
    That’s my favorite q.

  35. Many answers. The simplest though is that it’s about 10 feet because all rivers are about 10 feet wide. Simple.

  36. The other Matt says:

    When i read “river” i thought it will be a question including a boat acrossing this river and will be drift down….

  37. Jethro Stevenson says:

    I have an idea, however I suspect there is probably a simpler method as mine involves climbing the tree.

    As for Derren questions, I’d be really interested to know whether he is fully documenting his routines for future magicians. There are so many great routines which have passed into history with only theories on how they were achieved and although there is something to be said for leaving an air of mystery behind, it would be a sad loss if his techniques dwindled to myth.

    and if he is documenting it, I’d be more than happy to look after a copy for him 😉

  38. Please ask Derren this question: “Imagine that you are standing on the banks of a straight river…”

  39. JohnLoony says:

    Why would I want to know the distance to the sign in the first place? What does it say on the sign?

  40. Anonymous says:

    Wish I could think of a question for DB but I guess w/o a ticket I’d never know the answer anyway right?

  41. THe Masked Twit says:

    I’ve been struggling with this one. I got a rough answer yesterday within about a minute, which I thought was OK.

    Then I noticed people were reporting having two solutions.

    Went to the gym this morning – which is sometimes a good place to think – got a second solution and then almost immediately afterwards I thought of a third way to solve it. (eat your heart out Tony Blair).

    If I’m allowed to “imagine” that I am a bat (the question doesn’t specify what kind of life form one is to imagine one is) I can also think of a third way.

    Also thought of a great idea for a song cycle in the gym – but that’s probably way more information than you guys need.

  42. Naoko Fujimoto says:

    I cannot be solved. However, it is possible to measure the width of the river if there is a cap.
    I translate Japanese to English by Google.I am not good at English.

  43. One Eyed Jack says:

    Employ a family of Canadian beavers (If the river isn’t in Canada, you may have to import some. Beavers from other areas can be difficult to work with. However, I’ve found that Canadian beavers are quite amenable to direction, providing you reward them with a case of Molson. Anyway…) to fell the tree and build a damn upstream from you. When the water drops sufficiently, pace out the distance across the river.

    Really quite simple.

  44. AL says:

    Clearly this problem lacks a certain element which has been omitted by mistake, the first time this has ever happened on this blog. But there is always a first time….most probably while the Wise Man is preparing for an important interview….

  45. ph49 says:

    my first solution requires the use of a stick, some hinterland, and an uninterrupted view of the sign.

    my second requires the use of a watch, which I think is prohibited. So let’s stick with the first.

  46. Norfolk All says:

    Here are my calculations:

    If x is the length of the cloth, then x/3 + x/4 + 8 = x


    4x + 3x + 96= 12x

    and thus x=19.2 m

  47. Dave's mate Dave says:

    Hi, its Dave here. Have there been any messages left for me?

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