Math Wiz needed

I am horrible at math. When I do math I just do it over and over and it will click. I have been trying to figure this out for a week and can not grasp the concept of different percents of a grade. In med terms these are my grades and what percent thay are worth. Can anyone tell me what my grade is? Thanks in advance. Also an explanation would be good so I can try to do it my self and get the same answer.
Assignment .........% of grade ...............Total points .................my grade
Case study ..............15 ........................100 ............................88
case study ..............15 ........................100 ............................0
Discussion board .......15 ........................70 ............................68
quizes .....................15 ........................1200 ........................1085
Midterm ...................20 ........................100 ..........................91.5
Final .......................20 .........................100 ...........................95
Homework ................15 .........................100 ...........................50

thank you