Hint: Remember that each of them told the truth in one of the statements and lied in the other.
Answer: Brian robbed the Post Office.
We know that each of them told the truth in one of the statements and lied in the other.
So according to Alan, as one of his statements is false, it must have been either Derek or Brian.
If it was Derek then Brian's statements would both be true, which isn't allowed.
Therefore it was Brian, which agrees with Charles and Derek's statements.