Case 189
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.
Casey
“Quinn is guilty.”
Jules
“Parker is innocent.”
Taylor
“Casey and Parker are both innocent.”
Parker
“Jules and Taylor are both innocent.”
Quinn
“Casey is guilty.”
Who is guilty?
Solution: Quinn
If Quinn is guilty, Quinn's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.