How Do I Deal with a Frustratingly Dumb Coworker?

Dear Friends! I have a coworker who, for lack of a better word, is really stupid. He asks me how to do the same thing several times per day, regularly makes inane comments, and screws up projects for the whole team. I only care that he's so dumb because he regularly affects my work, but obviously someone thought he was smart enough to hire. What can I do?