Case 018
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.
Alex
“Neither Harper nor Frankie is guilty.”
Lane
“Either Alex or Riley is guilty.”
Harper
“Lane and Frankie are both innocent.”
Riley
“Lane is guilty.”
Frankie
“Neither Alex nor Lane is guilty.”
Who is guilty?
Solution: Riley
If Riley is guilty, Riley's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.