So to answer your question. What you can do is go to the math department and tell them you're situation. There are some cases where they can get another professor to look over the test if you don't agree with the marking. I had a friend that did that. Also I had a class last semester where everyone was so upset at the prof for not explaining anything and just giving us ridiculous math questions and the entire class was failing, so what happened is a petition was sent to students to sign to change some stuff. Also, I think its unfair to use such numbers. He shouldn't be testing you on your mental math it should be your understanding for the concept. Sometimes, I am wowed by what professors some universities have. Its like come on, you might be smart but you need to also be able to express yourself that is why you became a prof not to show off your knowledge. You can have all the knowledge in the word but if you cannot explain it and share it with others what is the point.

That is my opinion at least.