It’s the Friday Puzzle!

76

lionYou are in trapped in a room that has two doors.  One is an exit and the other has hungry lion behind it.  There two people in the room with you. The first is a nice man who always tells the truth and the other is a second hand car salesman who always lies. Both of these people know what is behind each door, but you do not know who is the nice guy and who would sell you a junk car.

You may ask one of the men just one question.

What should you ask in order to escape the room and avoid the hungry lion?

As ever, please do not post your answers, but feel free to say if you solved it and how long it took.  Answer on Monday.

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76 comments on “It’s the Friday Puzzle!

  1. Chewitt321 says:

    This is a good one Richard!!

  2. Adjay says:

    This is amazing puzzle..cant wait to see the answer..LOL

  3. thirdman says:

    To paraphrase Terry Pratchett: “Which door is the lion behind? And before you think you’re mister clever by lying, I’m going to be throwing you through first.”

  4. Frankie says:

    Love it! This is a classic puzzle that I know the answer to, but find it difficult to explain!! Anything that can be found in the film ‘Labyrinth’ however gets my vote 😀

  5. Nick Sharratt says:

    Too easy as it’s such a well known format for a puzzle, so about 10 seconds just to check I’d remembered it correctly.

    I like thirdmans answer though 😉

  6. ZenMonkey says:

    I do know the answer to this one and can explain it, but only because I’ve played way too many text adventure games in my life. I like this puzzle because it’s so elegant.

  7. Richard Strand says:

    I love this puzzle, but it only took me a couple of seconds as I’ve heard a similar puzzle before.

  8. Mark Roberts says:

    About 10 seconds. Love this kind of logic puzzle though.

  9. Adam Short says:

    An oldie, but a goodie. I already knew the answer, but it’s nice to see this one again.

  10. Sander says:

    This has to be one of the most famous (or oldest) of these kind of riddles. Still had the answer in my cache, so it took me only a couple of μs to solve it 😉

  11. c00 says:

    Meh, already knew it 😦
    Fun one tho.

  12. Blake says:

    I’ve heard this one before as well, but it’s a good logic puzzle, so it’s nice to hear it again.

  13. i knew this answer in -5 sec because i remember it from the movie labyrinth, this is a classic!

    • Lux says:

      Ah, I knew I’d heard it somewhere before. That film was disturbing, in the sense that Bowie’s crotch was disturbing.

      Anyhoo, I’d completely forgotten what the solution is but figured it out once I got past considering just asking each of them which door to go through. Obviously that doesn’t work so it’s a matter of deduction really – what else could you possibly ask them. There’s little else of relevance that you could ask before stumbling across the answer.

      Took me a good half hour to figure out but that’s because I was watching the film Election and was very distracted.

      I do like this puzzle, after a series of not entirely satisfying puzzles.

  14. Fran6co says:

    Idem : already knew this one.
    … Gotta watch Labyrinth again, can’t remember that in the movie …

  15. Dan says:

    I’m not sure I have enough information to find the one single solution.

    question: how did you get the information about the two? or the lion for that matter? because if one of them told you, wouldn’t he be the truth teller?

    second: does it matter which is more trustworthy? surely they can put their professions aside to find a way out of the room as they are clearly in the same predicament as I. if not, why would a nice guy do this to me? shouldn’t he simply open the right door and let me out?

    Thirdly: Do you have to use the two to figure out the right door? I think I could tell which had a lion behind it given it’s hungry growling, and if not, it was probably asleep so why can’t I just try both?

    Foruthly, do I have to ask a question? can I instead say “gee, I sure with I could drive right out of here, that reminds me, I need a new or used car” and watch their reactions?

    and finally, do I know which door I entered from? if so, given there are only two doors, it would be pretty obvious.

    I do however know the traditional answer to this puzzle. I just think it’s cliche’d and so I have no problem pointing out plot holes

    I think basically after picking a door I’d switch because that would give me an extra 33% extra probability of getting the car over the goat

  16. Mark Probst says:

    Old one. Different version: There’s only one guy in the room and you don’t know whether he’s always telling the truth or always lying.

  17. Christy says:

    Got it! Took me about a minute. Interesting puzzle.

  18. I’m relion on him lion ’bout the lion!

  19. DavidW says:

    Two seconds – isn’t this a slightly modernised version of a REALLY old philosophical puzzle?

  20. A classic. I recently discovered a trove of hundreds of this class of puzzle here. Also, if you think this one is too simple, try out The Hardest Logic Puzzle Ever.

  21. woupiestek says:

    Been there done that…

    What if there is an unreliable third man in the room, that sometimes tells the truth, but sometimes lies, and if you get to ask a second question?

  22. John Pilkington says:

    I remember that puzzle when I was growing up and Tom Baker played Doctor Who. The timelord had to solve the same problem. Coincidently he came up with the solution in 59 seconds.

  23. livedman says:

    I wouldn’t have got it if I hadn’t seen Labyrinth.

  24. Vern says:

    Diabolical. And quite handy. I found myself in just such a situation only last week.

    PS – Can the lion have fricken’ laser beams?

  25. Ois says:

    Got it, right away. There is a way of solving it without asking what will happen I believe. If not, I’m Lion food.

  26. Bletherskite says:

    About 10 seconds but coincidentally I had been looking at this riddle yesterday along with a few others so my memory was refreshed.

  27. dunno says:

    I don’t know the answer, but if you went up to one of them and punched them in the face and then asked of it hurt you’d know by the reply if they were telling the truth or not.

  28. Nopke says:

    Ehehehe …

    I came close but not completely .. so I googled … and found a similar one within a second .. and now know the answer.

    People are never that predicteable ofcourse.

  29. Niki says:

    In theory you forgot to mention that both men know of each other that they know the answer.

  30. Eulenherr says:

    To take this to the next level:
    http://xkcd.com/246/

  31. Annette says:

    I know, but I’ve known this kind of riddle before, so I didn’t even have to think about it.

  32. Samu says:

    Hehe, like a few others, I already knew this one 😉
    It…reminds me of the babe…

  33. Stuart says:

    I got my answer fairly quickly, but it does require an assumption that isn’t in the text. That being that each of the men knows the nature of the other man (i.e the truth teller knows the other always lies, and the liar knows the other always tells the truth). Is this a fair assumption to make… ?

    Always think something is not quite right when I have to make an assumption that isn’t stated 🙂

  34. Andy McGregor says:

    I agree with Stuart on the assumption, although I think it’s fair to make – after all, you can start asking how you get there if there are only two doors and all sorts!

    I think an interesting way of remembering these is using a sort of logic. For proposition, P:
    P = NOT P
    P = P

    Therefore need to combine to get same result, regardless:

    P = NOT P AND P = false
    P = P AND NOT P = false

    Indicating that asking either one what the other will say will give an answer that is opposite of the premise (e.g. “Will A say B is good” gives “no” if it is and “yes” if it’s not).

  35. Jerwah says:

    Too easy before I finished reading the puzzle

  36. Francesca says:

    Think I’ve heard this one before, so I had a solution fairly quickly.

  37. Alan says:

    Never seen labyrinth, but I think I’ve got it. Also, why are there so many typos in this one? “There” rather than “There are” and “the other has hungry lion”, not “a hungry lion”?

  38. Benjamin says:

    At first I was disappointed that you had just “re-packaged” a classic riddle, but then I re-read the setup and came up with an alternate answer that goes a bit outside the box.

  39. This is so old it’s a cliche. But a funny thing about that:

    I have a book of puzzles on my shelf, many of the contents of which would be suitable for this series (though most assume some knowledge of physics). Most of the puzzles in the book are unlikely to be ones the reader has seen before, yet the puzzle that the authors chose to place on the inside blurb is the one puzzle that the reader has almost certainly heard before.

    “You are a prisoner with two guards … “

    Why they would do that, is a puzzle in itself.

  40. MarkH says:

    Thanks to a Dr. Who ep from the 1970s, I know this one. 🙂

  41. slightly skeptical says:

    Oldie but a Goodie.

  42. Emma G. says:

    Thats easy just yell, “The tax man is coming” and follow the guy running for the exit!

  43. DeepField says:

    Knew it already, again.

  44. This one is probably over three thousand years old, however, in clever disguise.

  45. m5 says:

    I enjoyed listening to Karl Pilkington solve this one on an old XFM episode of the Ricky Gervais show.

  46. Ross Bagby says:

    This is an oldie (even the Tom Baker DOCTOR WHO once did a version on PYRAMIDS OF MARS — not to mention Cary Grant in CHARADE.)

  47. ScreamingGreenConure says:

    I read this one just today. however, I am too full of alcohol to do it. I want to hug the lion instead.

  48. Eh, feed the used car salesman to the lion. Problem solved.

  49. Jane says:

    I loved this one when I first heard it and I still think the answer is quite clever.

  50. phylliskirigin says:

    Yes, I’ve heard a variation of this before, too. It’s still fun to let your mind wander through the intricacies.

  51. deep says:

    I think i solved it after 14 hours.Usually i open your page at night and check my emails.I did many scenarios asking many questions to solve it but i fell asleep.After a good night sleep and a nice breakfast ,it hit me and i found the answer.I hope the question will be right , i just have to wait till monday to know.. thank you Mr.Richard.

  52. Mchl says:

    I like the variation of this puzzle as presented in xkcd comic #246 (Labyrinth Puzzle)

  53. uksceptic says:

    Classic.

    Labyrinth is a great movie.

    You remind me of the babe…

  54. Matthew Gress says:

    I also heard a variation from my Russian text book where there were 3 persons, the god of truth, the god of lies, and the god of diplomacy (who told you what you wanted to hear, apparently) and you could ask 3 yes/no questions to solve that one. It is similar to “The Hardest Logic Puzzle Ever” (though that is a matter of opinion, I am sure, http://en.wikipedia.org/wiki/The_Hardest_Logic_Puzzle_Ever).

  55. Michael K Gray says:

    I solved it even before I had fully read the blurb, but I cheated, as I was already very familiar with a variant of this classic conundrum…

  56. doug says:

    I solved it. Took a matter of a few seconds. I heard this question about twenty years ago, so that’s probably why I knew the answer quickly.

  57. Ginger says:

    I’ve already heard the puzzle, so I knew the answer right away.
    This is kind of off topic, but did anybody else see the lion’s teeth as candy-corns?

  58. Brady says:

    Doesn’t count, since I knew the answer from too many sci fi and fantasy stories. But I prefer the variant that has a third brother who lobs the head off anyone that asks tricksy questions.

  59. Lorena says:

    I asked the good man what would be the aswear if I ask the man who lies which door It’s the exit?

    • Lux says:

      If you already know which man is which then you can just ask them directly. I think the point is that you don’t know which is which.

  60. Martinus says:

    Recognised it instantly and even remembered the answer. I also think the Pratchett version is good.

  61. Tom Rhoads says:

    I recognized it, but had to talk it through with my wife to make sure I remembered how to do it. Straightforward logic once you know the answer. 🙂

  62. Jeremy Henderson says:

    C’mon, Richard, everyone know this one. What’s next, the riddle of the Sphinx?

  63. Dr.Used Car says:

    I am amazed with it. It is a good thing for my research. i have a blog about used cars too, please come visit my site when u have time ^_^

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