First, the big news is that the website for my new book, Paranormality, has just launched here and contains a nice little video to put a chill up your spine.  The book can be preordered from Amazon at 50% discount here.

UPDATE: Paranormality now the top selling popular science book on Amazon – thank you all so much!

Second, there is lots happening over the next few days.

– On Saturday I am speaking about the psychology of self-help at the School of Life.

– On Monday I am speaking at the Sheffield Skeptics in the Pub.

-On Tuesday I am talking about jokes at the British Library.

OK, here’s the puzzle.

What is the missing number (oh, and the 7 is not a typo)?

As ever, please do NOT post your answer, but feel free to say if you think you have solved it and how long it took.  Solution on Monday.





    1. Oooh. Was so confident in my answer I didn’t even finish the number line… Then I saw your comment…

      So either I am wrong, or the puzzle is wrong. I suspect I am wrong, and this is more subtle and complex than I first thought.

    2. Absolutely. Took me <30 secs but then that 7 screwed me up. If the 7's correct then can't see pattern at all.

    1. I suspect that most of the people who said they got it quickly didn’t check their solution carefully.

  1. @Pseudonym: It’s really weird that our theory would work for the whole series but the last number… and that we’d be off by 1. I mean what are the odds?

    But you’re right, I suspect the others didn”t check their solution carefully either.

  2. Yeah if the last number were 8 that would be freaking trivial. But it isn’t. The obvious one also doesn’t seem to have anything to do with the arrows, so there might be something triiiiiiicky going on here.

    1. I think my theory accounts for the arrows pretty well actually. But maybe we’re not talking about the same thing 😉

    2. hahaha, how does my brain work when I had to go back and look for you’re pattern expecting an 8 🙂
      The other pattern (that works on all of them) seemed more obvious to me… which I honestly find really strange right now.

  3. Yeah, had the ‘easy’ (but incorrect) one in about 20 seconds. With another 10 seconds before I was disappointed. Getting to the real aswer took about 2 minutes more.

  4. Got it!

    Got the “obvious” wrong answer immediately. Read some comments, realised my answer was wrong. Looked again, couldn’t do it. Hit refresh a few times, read some more comments, tried again. Couldn’t do it. Hit refresh a few more times, and eventually got the answer. I don’t think any comment gave it away for me, but I am happy that I have the right answer now (after 10 minutes or so).

  5. Yep. Immediate (wrong) solution in less than a minute, correct solution (i.e. a simple one that works) in another minute.

  6. Trcky indeed thanks for the comments, otherwiese i would not have read to the end and have the wrong answer. Now after 6-7 minutes i have an answer which fits in the whole chain.

  7. By coincidence, I solved this puzzle just a few days ago. I guesstimate it took me about 30 minutes. Maybe it should be pointed out that the 7 is not a typo.

  8. saw the pattern immediately, took me about 30 seconds to verify it and calculate the answer.
    (it usually takes me much longer and i’m terribly frustrated to read other people commenting that they found the answer in 2 seconds…)

  9. Clever. Got the right answer after about 2 mins. Then read the comments and just had to go back up to figure out the wrong one!

  10. got it got it got it! ten minutes. I’m so chuffed (although I usually get these quicker, I was fooled by the ‘obvious’ answer)

  11. Got it. Took me 15 minutes of useful time and had to use pencil and paper, but got it. Head was not enough. Wonderful puzzle.

    1. Wow! I wrote the first thing that come to my mind, without seeing any comments (to avoid contamination), and my “got it” repeat the last 2 comments? What are the odds of that? The world is indeed small.

    1. All weekend off and on this thing has bugged me….came up with a number not sure how I got it went back and forth, forward and reverse… so I will see in a little while….

  12. Sneaky! I ‘got’ the obvious solution within a few seconds, then did a spot of maths to verify.

    As for the ‘real’ answer, I’ve got one that involves the same mathematical operation, but treats each number slightly differently to convention.

  13. I tried during arround less than 5 minutes and I found the correct answer and I don’t need to change the 7 on the right side by an 8.
    Can I say that the answer is a number with 2 digits?

  14. At last! I got the obvious (wrong) answer as I was reading it. Then once I realised it was wrong it took about 5 minutes of trying to get the wrong answer out of my head before I got the correct answer.

    1. Brilliantly put Emlyn

      “At last! I got the obvious (wrong) answer as I was reading it. Then once I realised it was wrong it took about 5 minutes of trying to get the wrong answer out of my head before I got the correct answer.”

  15. Well I have got a solution that works but it is not particularly elegant so I very much doubt it is the one we are supposed to get.

  16. That was a nice puzzle. Took me about 2 minutes to find the wrong solution and probably almost 10 more for the right one.

  17. It has only taken me 2 or 3 hours to not get the right answer. Took about 2 or 3 seconds to get the wrong one. I’m not as clever as the rest of you!

  18. Ha ha!!
    As the rest of of the peopole here… I first find this really simple but wrong solution – then this even simpler and correct solution! Nice!

  19. Got it. Looked at the decoy for a few minutes, got stuck, so I came back after a while to look at it again and it immediately clicked. You just need to put the ‘8 solution’ out of your head completely.

  20. Well, it’s taken me a couple of hours on and off, and one number keeps popping into my head. I have no idea why, can’t work it out, but I think it’s the right number. Like a few others started out on the wrong track. I’m certainly not finding it easy….

  21. I have no idea about this one, have been looking at it for an hour or so and think I will have to come back another time. I am also drawn to one number but am not sure of the reason. I will have to keep thinking..!

  22. Got it !
    It’s a VERY GOOD one ! If I hadn’t read the “7 is NOT a typo” I wouldn’t have got the answer right !
    Same as Robert Johnson: obvious but wrong answer in about one minute, correct one another minute after realizing the first one was wrong !

  23. where fast on the wrong to the “8” solution gave up for a time, read comments and found the right answer after reading “mittfh” ‘s comment…

    The hardest Part is to get the “8” solution out of the head…

  24. nice……………………………………………………………………………..^_^b




  25. Very cool. I wonder how much work went into the misleading pattern?

    I spent a minute or two on the obvious but wrong solution, then another couple of minutes until I was able to think sideways enough to get the right angle. Maybe five minutes total processing before I had the answer.

  26. Took me about a minute, but I was surprised to stumble on a working solution that quickly. Discarded the incorrect pattern immediately thanks to Richard’s hint.

  27. 30 seconds for the carefully designed wrong answer, then 5 more minutes “trying”, 1 minute to realize that I had to make the “7” work, and then 20 seconds.

  28. finally…
    I thought i was a dumb nut not figuring it out. got the wrong answer first, was stuck that operator.

    VERY tricky one, especially working in such a confusing pattern must have taken quite some time!

  29. Although pretty simple, I like this one. It’s kind of like an illusion… you see two different things before figuring which one is the ‘intended’ image.

  30. Six or seven hours (on and off) hard thinking and I have managed, at last, to still not get the answer. Getting ready to kick myself when the answer(s) are published!

  31. This one was easy, got it in about 10sec!… the part i don’t understand is why will i think that the 7 is got to be 8?:s

    1. The answer you have isn’t the real one. There’s a solution that fits all the numbers, including the 7.

  32. Haha, as everyone else, was very quick with the “wrong” solution.
    Took me about 10 minutes to complete it after mittfh said they treated the numbers differently than normal to find the answer.

  33. Got it in less than a minute ~ but my brain is wired for numbers/math/patterns.

    No idea why.

    the written word? Dyslexia stinks sometimes.

  34. kept thinking along the right lines but – as usual – assumed it had to be more complicated and only got the right answer after about 15 minutes (with the first 5 minutes spent on the wrong answer that everyone else has got).

  35. By the way, it’s good to see a puzzle that actually exploits the brain’s ability to seek patterns that aren’t there, and challenge standard assumptions – so much better than trivially bogus and incorrect pseudo-statistical problems. Well done

  36. YAY didn’t see anything on first glance, then read the comments, got the correct answer after about 30 seconds of looking at it again, then wondered what all the fuss about the *wish it was an 8* comments, then saw how easy it was to confuse the two!

  37. Got the correct answer immediately. I haven’t gone back to puzzle out why so many people thought the 7 was a typo though.

  38. Got it in four minutes. I think it will be correct. If so, then this puzzle is a little bit nasty and will get many, many comments on monday !

  39. Aargh! Looked at it this morning and got the “obvious” solution that would require the last number to be 8, then came back to it this evening after some beers and the correct solution jumped out at me. If anyone is having problems solving this then I recommend a few pints of Tanglefoot 🙂

    1. Well, I had a couple of glasses, and I decided to invoke my own rule to account for the “blip” of the last iteration, justified by the fact that it’s the only single-digit number.

      I will call that sufficiently parsimonious. Who’s gonna say I’m “wrong”?

  40. 4 or 5 minutes – so easy to get hung up on the obvious wrong answer it’s easy to imagine that the right answer must be a very complex formula…

  41. argggggh!!!

    I got it in my second try. And no, the number 7 is correct. 🙂

    About 5 minutes in the correct anwer, seconds in the bad option.

  42. Easy. Got it in under a minute.

    I don’t see why the last number would be 8 – maybe it has to do with the fact that I figured it out by reading the sequence backwards.
    I think the obvious-but-wrong solution has escaped me this time. (it’s a first!)

  43. ARRGGH – Can’t believe it took so long to get the answer. Probably about two hours on and off. Couldn’t get the wrong answer out of my head. So simple when you get it.

  44. Had to run through about five different hypotheses before getting it.

    Very neat, very sneaky. A puzzle with a sort of trapdoor that drops the mind onto the wrong track.

  45. Like most, my first answer was the red herring pattern. Spent another 10-15 minutes on it to arrive at a solution that works for the whole puzzle.


    After seeing the 7 vs 8 problem, I immediately convinced myself that ‘nature’ of the solution would not be along the same lines as the red herring pattern. I knew I would have to (as Timothy Mills said) think sideways. So I started on the right, rather than the normal left-to-right reading.

  46. Oh, brilliant!

    I, like so many others, got the wrong answer immediately. It made me all frustrated, so I decided to close the page, clear my head, and try again the next day. And I got it!

    The ones you have to work a little harder for are always the best.

  47. the trapdoor comment is spot on! i had a hard time climbing it out of it until a friend pointed out looking at it from a different, more adventurous perspective!

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