Most schools have ranges for the grades they assign, although most Ethiopian schools evaluate out of 100. e.g. 85 and above might be an A, 80-85 might be an A- and so on. You can convert your scores out of 100 to the letter grading system. Then, add together all the values you get from the grades. Finally, divide this by the total number of subjects. A=4.0 A-=3.75 B+= 3.5 etc.