Home > Logic > Liars and Knights
| Knights, Liars, and Spies (Posted on 2022-02-12) |
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On a remote, mysterious island there are three types of people: Knights, who always tell the truth; Liars, who always lie and Spies, who may answer in any order that they think fit.
Lenny is visiting this island when he comes across three men. He knows that one of them is a knight, one is a liar and one is a spy. However, he doesn't know the exact identities of the three men.
Lenny is permitted to ask these three gentlemen precisely two yes-no questions. All the three men will answer each question, one at a time, with either a "yes" or "no" response.
How does Lenny determine the true types of these three men?
*** Try avoiding extraneous questions like "Is the sun shining?" or, "Is the earth flat?"
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Submitted by K Sengupta
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Rating: 5.0000 (1 votes)
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Solution:
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(Hide)
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At the outset, Lenny should ask each of the three gentlemen: "Are you a spy?"
The responses would be:
Knight: No
Liar: Yes
Spy: Yes, or No.
If there are two "No" responses, then the gentleman replying "Yes" must be the Liar.
If there are two "Yes" responses, then the gentleman replying "No" must be the Knight.
After identifying the inhabitant as liar/knight, Lenny now points to one of the other two and, asks him this question: "Is he a spy?"
Based on the false/true response from the liar/knight he can immediately figure out the spy.
The remaining inhabitant must then be a knight/liar.
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Also refer to a slightly different solution submitted by Kenny M in this location. |
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