While a number of news outlets have reported on Beau, the dog who can do math, the Missoulian is one of the few to have a complete story and video up (which is at the bottom of this article). The owner of Beau claims to have taught him how to do simple addition and subtraction as well as a few story problems and even scoring for some games. Beau just indicates his answers with barks.
Through the years, I’ve read of plenty of examples of stories like this that turned out to be hoaxes. The owners were signaling the animal when to stop. Sometimes, they were even doing so with by some sort of signal they weren’t even aware of. Beau can apparently give answers even when his owner isn’t around, lending more credibility to the story. While I’m still a bit skeptical, the video is entertaining and impressive either way.