S2·E8 Logic 101

It starts when Morgan bets a lunch that Blaire can't not date a girl for a week. Then, Mr Noseworthy asks Blaire if he can coach an Upper Upper Redwood High girl's sports team, but he declines. Eventually, his brain kicks in and he decides to keep it secret from Morgan. However, Mr Noseworthy has seen Blaire's grades, and says he needs to improve on them. He pays Roger to do it for him. Morgan hears about Blaire coaching A team, but Blaire covers it up by saying their senior citizens. Roger slips up on his own homework (Logic 101) and the truth is revealed. When Mr Noseworthy is about to give him the job, Sarah and Morgan trick Blaire by giving him two sets of homework from Roger. They ask him the question on the homework, ""One of us always tells a lie, but the other one only tells the truth. You can only ask us one question."" Failing, Blaire ends up back in his old habit. Speaking of which, what is the answer??