Who robbed the bank? Three suspects are arrested for a bank robbery. Suspect $A$ says he did not rob the bank. Suspect $B$ says he did not rob the bank. Suspect $C$ says suspect $B$ did not rob the bank. If $A$ is telling the truth, then that means $B$ and $C$ are lying, which means that $B$ robbed the bank. Is this correct? EDIT: Sorry for not including this, the restriction is only one is telling the truth.

Given the assumption that only one suspect is telling the truth, the solution can be established in one fell swoop: If B is telling the truth, then C is also telling the truth; this would violate the assumption, so B must be lying, and hence he is the robber.

You can, of course, consider all the other possibilities as well and rule them all out, but you don't need to.