## Thursday, March 18, 2010

### Puzzle - find the safe path

From www.ft.com

Two guardians watch over a fork in the road. One path leads to safety, the other to a grisly death. One of the guardian is a knight, meaning that he always tells the truth. The other is a knave, meaning that he always lies, answering “yes” to questions whose correct answer is “no”, and vice versa. You do not know which guardian is a knight and which is a knave. With a single question, how will you find the safe path?

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woodpecker said...

I think this question should work :
"Which path do you think the other guardian would point me if I want to remain safe?"

Both of the will point to the same path, which is the WRONG path.

That's because if the knight will know that the knave will lie so he will be telling the truth by pointing to the wrong path. For the knave, he knows that the knight will point to the correct path, so he will lie and also point to you the WRONG path.

Since there are only 2 paths, just take the other path that both of them did not point to. That is the SAFE path.

Anonymous said...

Approach either path and ask its guardian:

"Will the other guardian tell me yours is the safe path?"

If the guardian answers NO, then this is the safe path.
If the guardian answers YES, then take the other path.

The question forces the same answers from Knight and Knave, regardless of which path they are guarding.

cheapskate said...

If you are "not the knave guardian", show me the safe route.

If the good guardian is on duty, he will show me the safe route.

If the knave guardian is on duty, he will answer "I am the knave guardian" and acts like the good guardian, and therefore show me the safe route.

Anonymous said...

"If I were to ask you YESTERDAY, would you say (point to one of the paths) this is a safe path?

The knight will always tell the truth.

The knave would have lied YESTERDAY. So he will have to tell the truth in answer to the question.