Case 203
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.
Reese
“Either Harper or Casey is guilty.”
Harper
“Lane is guilty.”
Casey
“Reese and Lane are both innocent.”
Lane
“Casey and Quinn are both innocent.”
Quinn
“Casey is innocent.”
Who is guilty?
Solution: Harper
If Harper is guilty, Harper's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.