Case 350
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.
Morgan
“Casey and Blake are both innocent.”
Casey
“Either Morgan or Emery is guilty.”
Emery
“Either Casey or Frankie is guilty.”
Frankie
“Morgan is innocent.”
Blake
“Neither Casey nor Frankie is guilty.”
Who is guilty?
Solution: Emery
If Emery is guilty, Emery's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.