77% or C+
13 - 4 = 9 correct 9/13 x 100 = 69% (rounded)
If by grade you mean percentage, you would have scored 67.5%. 40-13= 27 (27/40) * 100
10/13 right = 77% grade
You use division. For example 13/47 is approximately 0.276 or 28%. 100-28 is 72 so the grade would be a 72.
77% or C+
12/13 correct is a 92% which is an A-
54% or F
84.6 around that
2 wrong out of 13 means 11 right out of 13 11/13 = 84.6% This typically translates to a solid B on many, if not most, grading scales. At Hogwarts it would probably be an A (Acceptable) - or possibly E (Exceeds Expectations) YMMV
8/13 right = 62 %
37.14%
That would be a grade between 86% and 87%.