The 20% weight is for the average of the best 80% of your problem sets' scores.
(These standard weights were modified at semester's end to put greater emphasis on your mid-semester exams if the average of the best two was greater than your Final Exam score, but to put greater emphasis upon your final Exam score if that was greater than the average of your best two mid-semeser exam scores. See the Grading Method page for the details..)
Grading scale
A
A-
B+
B
B-
C+
C
C-
D+
D
F
86
82
78
75
72
68
62
58
55
50
<50
Example calculation of a course grade
Score
Weight
Net score
Average, best two of Exams 1–3*
81
50%
40.5
Final exam
72
30%
21.6
Average, best 80% of problem sets
70
20%
14.0
Total score
76.1
Grade
B
* For example, that average would be 81 if your mid-semester exams' scores were 84, 60, and 78, since the low score of 60 would be dropped.