Going back to what Wraith and MikeMikeMike said, it can't be logical to think that the girl killed her mother because the mother was having an affair with the man at the funeral. For the question says that the girl knows nothing about him- so she can't know that he was having an affair with the mother. So, contrary to what the police seem to think, the only logical answer is that the girl has figured out that, since the man attended her father's funeral, he might also attend her mother's. If the police really use this question to decide who's sane and who's insane then perhaps the people they've locked up are actually normal and it's the rest of us who are psychopaths?