In a remote island there are only two types of people that live on it, Knights and Liars. Knights always tell the truth. Liars always speak falsely.

Five suspects are interrogated in connection with a murder. Their statements are as follows:

Al: "Cal and Dan are lying." 
Ben: "Al and Elmer are lying." 
Cal: "Ben and Dan are lying." 
Dan: "Cal and Elmer are lying." 
Elmer: "Al and Ben are lying." 

Which suspect do you know for certain to be a liar?

If you can know only one liar for certain it has to be Al since he is the only one who doesn't accuse someone who also accuses him.  This is an argument from symmetry not logic.

Figuring out the logic is hurting my brain so I'll stop here.

Posted by Jer on 2013-02-26 12:28:12