Dude, get real. My average grade is 8.4, and that's quite good actually. Don't go whining about an 80%, it's pretty good.
I don't know math tests are like at your university, but in my experience 6-7 hours of studying is a regular amount and a normal grade for such work. I usually study for the exams of a subject for about 10-15 hours and I tend to get 8's and 8.5's primarily. That's simply reality. If I'd study 20 hours I wouldn't get a much higher grade, I simply make mistakes both small and large on tests.