The average score on the assignment report for a student is the average of the scores they received on each individual problem. For example, on a 2 question assignment, if they received a 33% on problem 1, and a 66% on problem 2, their assignment score would be 49.5%.
A class score is the average of each student's assignment average.
The score a student earns on each problem depends on the number of attempts, hints, or explanations used, and whether they eventually got the right answer on their own or had to click 'show answer'. See the Scoring rubric here for more information.
If a problem set includes open response questions, these will not be factored into the student's total score until they have been manually scored by the teacher. The teacher has the option to score on a four point scale and that will carry over to the assignment report and automatically factor into the student (and class) overall score.