Case 304
The Swapped Envelope
A sealed envelope was replaced with an empty one.
Rule: exactly one suspect is guilty. The guilty suspect lies. Every innocent suspect tells the truth.
Reese
“Drew is guilty.”
Rowan
“Drew is innocent.”
Marley
“Neither Finley nor Drew is guilty.”
Finley
“Reese is guilty.”
Drew
“Rowan and Marley are both innocent.”
Who is guilty?
Solution: Reese
If Reese is guilty, Reese's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.