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Two Guards Riddle. [11] (thelemmy.club)
submitted 1 week ago* (last edited 6 days ago) by [M] to c/Mimi@lemmy.world
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[–] 45 points 1 week ago* (3 children)

What always bugs me about this in popular culture is that you only get to ask one guard the question, not both. So the question you would ask here would be:

“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?”

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  • [–] 9 points 6 days ago* (last edited 6 days ago) (1 child)

    Wouldn't it be easier to just ask something like "is red a color?" To one guard?

    I think the original riddle also had 2 doors and the question was something like "which door would your partner say leads to freedom" iirc, which makes both point to the one that leads to death.

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  • [–] 16 points 6 days ago

    You only get one question to one guard. So if you ask ‘is red a colour’, you find out which guard is lying, but not which door is the correct one (or either of the guards’ credit card info)

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  • [–] 6 points 6 days ago (1 child)

    That's right, we've all seen Labyrinth

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  • [–] 6 points 6 days ago (1 child)

    You’d be surprised, given the replies I’ve had

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  • [–] 6 points 1 week ago (1 child)

    can you unpack the logic in that last part for me?

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  • [–] 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.

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  • [–] 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

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  • [–] 5 points 1 week ago* (last edited 1 week 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’.

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  • [–] 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?

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  • [–] 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.

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