320
Two Guards Riddle. [11] (thelemmy.club)
submitted 1 week ago* (last edited 6 days ago) by [M] to c/Mimi@lemmy.world
34 comments fedilink hide all child comments
 

https://www.pixiv.net/en/artworks/125076972

you are viewing a single comment's thread
view the rest of the comments
[–] 8 points 1 week ago (1 child)

“If I asked the other guard for a credit card number, expiry date and security code that wasn’t yours, what is an answer they could give me?”

Truth guard: the other guard lies => if asked for details that weren’t the truth guard, must lie => tells details that are truth guard’s => truth guard tells you these correct details

Lie guard: other guard tells truth => could state any details that are not lie guard’s => lie guard must therefore tell you their credit card info

So you get one of the guard’s info. You could change it to a specific guard’s info if you’d like.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •
  • [–] 11 points 1 week ago (1 child)

    I guess I'm not getting it. Couldn't the "that wasn't yours" part be misconstrued to allow for anyone's info?

    The truth guard could recite someone else's credit card (which would still be a win), but the lie guard could fabricate false numbers completely

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •
  • [–] 5 points 6 days ago* (last edited 6 days ago) (3 children)

    You could rephrase it to

    “If I asked the other guard for an example of a credit card number, expiry date and security code that weren’t the details of the guard I am currently talking to, what is an answer they could give me?”

    if you’re worried about misinterpretation.

    If you were to ask the lying guard for ‘an example of a credit card number, expiry date and security code that weren’t the details of the other guard’, they would necessarily have to give the correct details, which the truth guard would pass on

    If you were to ask the truthful guard for ‘an example of a credit card number, expiry date and security code that weren’t the details of the other guard’, they would necessarily not be the correct details. However, the truthful guard could answer with any incorrect details, and so the complement of the set of details the truth guard could give is only the correct details, which the lying guard would therefore have to give. That’s why you ask for ‘an example’.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 6 child comments
    •
  • [–] 2 points 5 days ago (1 child)

    If you were to ask the lying guard for ‘an example of a credit card number, expiry date and security code that weren’t the details of the other guard’, they would necessarily have to give the correct details, which the truth guard would pass on

    Couldn't they just lie and give you a non-working example?

    Or are you hinging your question on a variation of "don't tell me the other guard's credit card information", which you expect the lying card to contradict and tell you?

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •
  • [–] 2 points 5 days ago (1 child)

    You’re asking for an example of details that aren’t those of the other guard. If they give you a non-working example, then the lying guard has been truthful. Which is a contradiction.

    Consider the question ‘what is an example of something that is not a cat?’. Any answer that is not a cat is truthful; therefore the lying guard must give an example of a cat.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •