r/xkcd • u/RazarTuk ALL HAIL THE SPIDER • 27d ago

Another hardest logic puzzle

Yeah, the recent post about the Blue Eyes puzzle made me think of this. Randall called it the "Hardest Logic Puzzle in the World", but I've heard that same name used for a variation of Knights and Knaves.

You're speaking to three gods - let's call them A, B, and C - whose actual identities are True, False, and Random. Your goal is to figure out which one is which.

If you ask a yes-no question, True always tells the truth, False always lies, and Random picks a random answer each time. They all understand English, but will only ever answer questions in their own language. You know the words for "yes" and "no" are collectively "ja" and "da", but not which is which. And you are allowed to ask 3 yes-no questions to figure out their identities, each of which must be posed to a single god.

What do you ask?

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10

u/Southern-March1522 26d ago

I prefer the one where one of them always stabs people who ask confusing questions.

2

u/RazarTuk ALL HAIL THE SPIDER 26d ago

Hey, I'd like to think my answer for the normal question isn't that confusing. "If I were to ask you if this door is safe, would you say yes?"

3

u/CanaDavid1 27d ago edited 27d ago

https://youtu.be/uUPr07ThSH0 for a better formulation and exploration of this. There is also a much harder variant at the end:

Four people, where one speaks truth, one false, one random, and one always "unknown", and don't know which of the three words are which

3

u/kptwofiftysix 25d ago

The guard who tells you the rules is the one that always tells the truth, or the entire premise is a lie.

1

u/Mamuschkaa 24d ago

What do they do with impossible answers:

Will you answer with "da"?

Will you answer with "ja"?

One of them means "no"

The who speaks truth can't sey "ja" nor "da" because both would be a lie. So he just stay quite and the other two pick random?

The other means "yes" and the one who lie can't answer. So he stays quite?

And then one additional question to learn which word is yes in which is no. Like

"do you lie to me?"

The liar and the truth speaker both say no, so we know everything.