Case 244
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.
Jules
“Harper is guilty.”
Cameron
“Either Jules or Alex is guilty.”
Alex
“Cameron is innocent.”
Harper
“Either Jules or Cameron is guilty.”
Marley
“Either Jules or Harper is guilty.”
Who is guilty?
Solution: Jules
If Jules is guilty, Jules's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.