Case 312
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.
Hayden
“Emery and Finley are both innocent.”
Avery
“Emery is innocent.”
Jules
“Either Avery or Emery is guilty.”
Emery
“Avery is innocent.”
Finley
“Either Jules or Emery 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.