Psycho Teacher Grades Messed-Up Test

The teacher's name and the subject that he/she teaches shall not be disclosed, no matter how much you pester me to tell. If you are reading this story and you already know who this teacher is, then good for you. Go and gossip about it.
This teacher gave a test that had twenty-two questions. There were twelve questions on the first page and ten on the second. Read that over again, because it's important. Since the teacher was probably to lazy to take out a calculator, he/she asked us to pick two questions for extra credit. This would make each question five points. All of the questions on the second had two parts, so you could get part of the question right and get two-and-a-half points, which he/she rounded up to three. If you didn't understand this mess, don't feel bad. I'm still a little confused myself.
I got three questions and one half-question on the first page. Yes, you can get half credit on the first page. I got one question and one half-question on the second page. There were two questions that were not marked right or wrong. I picked the first two questions that were wrong for extra credit, figuring that it wouldn't hurt me. That leaves two questions wrong, and two halves, which will now be counted as one question wrong. That means there are three questions marked wrong, which, at five points per question, is fifteen points. I should have gotten an 85.
I ask the teacher to please explain how he/she graded the test. Again, if you don't understand this, I didn't either. Neither did the majority of my class. He/she picked the extra credit questions from the two that you got wrong on the second page, which he/she thought was harder. You also had to get twenty questions right for the extra credit to take effect. If not, you had only eighteen questions answered. Somehow. No one understands why. We just kept nodding our heads with no clue what was going on. (Then again, that's what happens in most classes, even if you can understand.)
If you have the slightest idea what in the world is going on, please tell me, because I'm lost in a territory that is far from my own.