Case 321
The Vanished Key
A locked evidence cabinet was opened without permission.
Rule: exactly one suspect is guilty. The guilty suspect lies. Every innocent suspect tells the truth.
Emery
“Frankie is innocent.”
Lane
“Neither Emery nor Frankie is guilty.”
Logan
“Either Emery or Lane is guilty.”
Frankie
“Neither Lane nor Marley is guilty.”
Marley
“Emery and Frankie are both innocent.”
Who is guilty?
Solution: Logan
If Logan is guilty, Logan's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.