Case 340
The Vanished Key
A locked evidence cabinet was opened without permission.
Rule: exactly one suspect is guilty. The guilty suspect lies. Every innocent suspect tells the truth.
Avery
“Drew is guilty.”
Alex
“Either Avery or Sam is guilty.”
Drew
“Either Avery or Parker is guilty.”
Parker
“Drew and Sam are both innocent.”
Sam
“Drew is innocent.”
Who is guilty?
Solution: Avery
If Avery is guilty, Avery's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.