ou are a logician in training for the police, and the time has come to take the certification test. The police chief brings you the test one morning, and says, "I must warn you, this is your only chance at the certification test; If you fail, you must keep training for another year before you can take it again."
           
- Five suspects were interrogated for a bank robbery.
             
- Each suspect was either a knight, a knave, or a liar.
         
- Knights always tell the truth.
       
- Liars always lie.
       
- Knaves strictly alternate truths and lies with each statement.
                   
- Police have evidence that suggests the perpetrator acted alone.
                   
- Police have evidence that suggests the perpetrator acted alone.

>During the interrogation, two questions were asked (consecutively) of each of the five suspects. Each suspect heard the other suspects' responses, and none of them made a statement between his or her two answers. Here are the two questions and their responses.

"Did you rob the bank?"
A: No.
B: No.
C: No.
D: Yes.
E: Yes.

"Who robbed the bank?"
A: E.
B: A.
C: l don't know.
D: E.
E: A.

The interrogators mentioned that something about their statements didn't seem quite right. The police chief adds, "The only hints I can give you are that C is not a knight and that there is only one correct answer. I'll be back in 24 hours to ask you who robbed the bank."

... as proposed by Penny and KennyM, where C is guilty because of the fact that the perpetrator acted alone and thus the robber is the only one who would know who did it. I would normally, and I thought of it myself before reading the comment list, but there are two factors that invalidate this solution:

- It is based on the assumption that no one but the robber knows who the robber is. This is not necessarily true; all or some of the other suspects could have been eyewitnesses. Without this assumption, all of AvalonXQ's proposed solutions seem valid (except for the possible typo he made concerning C being a knave if he was the robber).

- This puzzle is listed in Cryptography. I might be prone to ignore that fact based on a mistake by the submitter, but the puzzle was submitted by Dustin who is no newbie to this site and has even submitted other problems before this one to the Liars and Knights category! Since he is clearly aware of that category, why would he submit this to cryptography? The current solutions proposed have no justification for being in cryptography. Even assuming that it was a chance mistake, surely the journeymen who voted this problem in would have corrected the mistake.

Posted by Avin on 2006-02-17 11:12:27