I need some help counter-insulting someone.

/sarcasm "Oh, I'm so sorry; if you can't even do this, you're too stupid for me to help you."

"Oh dear, if you can't do this, it just might take a miracle for you to pass." /endsarcasm

All while putting on your best "business smile" (read: Fake smile often found on the faces of cashiers and receptionists).

p.s. Yes, that's two retorts. Use a sarcastic tone, but don't let it show at all that you're annoyed; if you look perfectly "honest" about it, he'll probably shut up. Make sure that you can actually do the math problem easily though; you don't want it to backfire on you.

No, don't do that. It doesn't work.

Just say "It sure is, Brett."