You are trapped in a prison and the guard decides to play a game.  If you win, you will be allowed to leave.  But if you lose, you die.   The guard says that he is thinking of number 1, number 2, or number 3.  You are allowed to ask one question to find out which of these three numbers the guard has in mind. However, the guard will only answer with a “yes”, “no”, or “I don’t know”.  What should you say to the guard?

As ever, please feel free to say if you think you have a solution and how long it took, but please do NOT post your answers.  Solution on Monday.  Have a good weekend!

95 comments

    1. Richard’s not a psychologist but a psychopath. The beginning is the same, but the end is very different! 😉

    2. lol …. how very true!
      I’ve thought of several things….one being “please, please, please don’t kill me!”… lol… I’m sure that isn’t the answer.

  1. I think the fact that guard seems to have a “God” complex by so nonchalantly wielding the power of life and death, and therefore his answer to my question is likely to be untrue (what am I thinking, no wrong, death it is then….). I’ll lead the other prisoners in open revolt and play better odds of survival, thankyou 🙂

  2. I think I have an answer. I’m not too happy with it though because it seems to make an assumption about the level of education that a prison guard has.

  3. A couple of minutes to think of 2 “answers”, neither of which feel likely to be the ‘right’ ones -one maths based and one logic based.

  4. I did this while driving to work, so road traffic distracted me, but I managed to get an answer in ten minutes. I think there’s clearly infinite different answers to this, but the idea is presumably to determine the required general strategy, which I think I have.

    I agree there is a small assumption here that the guard is numerate and doesn’t lie 🙂

    1. hey Faye I wonder what the guard looks like is he good looking….hehe… and if my answer is wrong do I get one final request? {I don’t smoke either…lol…}

  5. Phew, that was tricky. Saw the shape the answer had to be straight away but took about 10 minutes to work it out. 🙂

  6. I have one that works, but will almost certainly not be the official answer. 😛 It requires the guard to be honest and also have a good knowledge in genetics.

  7. I have an answer, but I think it might well be wrong.

    I assumed that the guard would be honest in his response and that I am not allowed anything as simple as:

    “say ‘yes’ if you are thinking of 1, say ‘no’ if you are thinking of 2 and say ‘don’t know’ if you are thinking of 3” 😉

    1. This was pretty much the cheesy answer I came up with too. Easy to rephrase that into a question. Feels like cheating. 🙂

  8. Hmmmmmm why should he say “I dont know”……..
    Im still trying to find the answer….

  9. “The guard says that he is thinking of number 1, number 2, or number 3. ”

    I don’t get that formulation … so, is it ANY three numbers (in a sense of: he is thinking of numbers A,B, and C) or is he thinking of THE numbers 1,2, or 3?

    confuses me! 😀

  10. I’m still working on an answer, or rather a question (10 minutes so far) but am moved to comment on that photograph of the guard, which is simply marvellous.

    1. Yeah but is that part of a trick? Cause based on one of our more recent surveys he may only be of use as eye candy…lol…..

  11. I haven’t got an answer yet, but what’s with that guard picture? Is this prison porn? I think if I were gay I might just stay in the cell with him.

  12. I have a math-ish question that took me about 5 minutes to come up with. Seems unlikely but I think it works…

  13. 5 minutes of sustained headscratching to arrive at watertight answer. No cheating or Googling this time. Nice puzzle.

  14. I guess I would have to ask, “Is that a regulation hair cut? Because it looks a little shaggy to me.”

  15. After thinking a while, I did come up with an answer that would probably work the way I want it to work, but it’s phrased in a fairly ugly way…

  16. Can we assume that the guard must answer truthfully? It’s not explicitly stated, and I can’t think of a way to solve it if he’s allowed to lie.

  17. Of the recent puzzles, this was the easiest for me – took me a minute or so with the initial hunch of what to ask a few seconds after reading. There are many variants to my solution.

  18. Umm, that guard looks like a stripogram… but I have one question and it took me about 5 seconds to think of it.

  19. Under a minute for the core idea. Crafting a question that actually implements the idea somewhat longer.

  20. I think there are several ways of doing this, although, maybe not very elegant any of them.

    Nice puzzle…

  21. I have an answer (or question rather) and unlike a lot of other people posting I feel pretty confident about it…..*smugness*

  22. This is a bad puzzle (only the second bad one that I can recall). There is a direct mapping of 3 number/answer pairs, so the only thing to do is make up a suitable question. The computer scientist in me got *a* solution immediately, but there are an infinite number of questions that would serve as solutions. Maybe a better puzzle would be how *short* a question can you think of that saves your life?

  23. I have an answer that I believe provides the solution. Took about a minute. Of course, it’s maths/logic related because that’s my area of training.

  24. I don’t recall a prison guard amongst the cast of the Village People, but that was a long time ago…

  25. By Jove, I think I’ve got it! In just a couple of minutes or so. I’ve got to start timing myself. I’m usually pretty horrid with games like these but I’ve answered a couple pretty easily.

  26. Got it in about 10 minutes.

    The breakthrough was realising I needed to utilise all three possible responses.

  27. I have an idea of what a good question might be, but if the guard’s mathematically challenged, it might be hard…

    I’d probably have to phrase it carefully so that even the guard would understand. It’s a given that he can’t lie, right?

  28. Had a little think, is the answer perhaps:

    ‘Is the number you’re thinking of less than 2?’

    A ‘yes’ answer would mean it had to be 1, ‘no’ would mean 3, ‘don’t know’ would be 2 perhaps? Just a guess!!

    Emma x

    1. you mist the last paragraph I will copy it for you.

      …As ever, please feel free to say if you think you have a solution and how long it took, but please do NOT post your answers….

  29. Guess what? Some of us have created a TEAM ANSWER. We laughed, we cried, we got distracted by cartoons and butterflies – but in the end, we did it. If it’s not right, our new friendship will be shattered and we will be broken individuals.

    1. Okay, my buddies want you to know that the butteflies are all in my bedroom (in pupae form) so did not distract them, just me, and the cartoons were inside my head, and so it was just me distracted by those too.

    2. She distracted us from distracting her. Be glad, we still wouldn’t have solved it if she hadn’t. Thank you, butterfly pupae. We owe you our freedom.

    3. Well SGC lives…I was wondering if you got lost in the future trying to find the answer…lol…with all the distracting going on did anyone get distracted by the guard? Remember your life is in his hands….lol….hmmm….now that’s a thought…… ;}

  30. Hah, after a few minutes wondering if the problem *might* be related to the famous “Monty Hall” problem, I abandoned that path and asked the guard a tricky question…
    (I assume that the goal to seek is to not get killed?)

  31. Oh wow, I have run through everything I can think of and none of them are passing the test. I don’t know. Infuriating!

  32. The guard is actually thinking?! Well, that’s something new.
    I might have an answer, but it probably isn’t the right one.

  33. The question would be,
    Is the difference between your number and the number 1 an even number?
    If he said yes, then the number is 3 (3-1=2) even
    If he said no, then the number is 2 (2-1=1) odd
    If he said I dunno, then the number is 1(1-1=0) neither even or odd (dunno)

    This answer can’t be proved wrong 😛 I hope 😛

    1. Oh yea!! But may be the guard is bad in math, and he would say “I dunno” :p
      btw, where should I post my answers, if not here?

  34. “When you divide the number you’re thinking of by that number -1 (x/x-1), is the answer greater than 1.75?”

    3 – If the number is 3, the expression is 3/ (3-1) = 3/2 = 1.5 – answer ‘No’ (NOT greater than 1.75)
    2 – If the number is 2, the expression is 2/(2-1) = 2/1 = 2 – answer “Yes” (greater than 1.75)
    1 – If the number is 1, the expression is 1/(1-1) = 1/0 = undefined – answer “I don’t know”

  35. How about this:

    “If I guess incorrectly, would your number be larger than mine?”

    If “yes” then hypothetical guess {1,2} => 3
    If “I don’t know” then hypothetical guess {1,3} => 2
    If “no” then hypothetical guess {2,3} => 1

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