How weighting is used to calculate grades is not always clearly understood. This explanation should be helpful. In the example, there are three weighted categories: Homework, Tests, and Final. Each is weighted 30%, 50%, and 20% respectively.
The figure to the right illustrates that a student's average for the Homework and Tests categories is 60% and 70% respectively. Since there is no score for the final exam, that category will not be used in calculating the student's average. The weight of each category used is multiplied by the student's average for that category. Those products are summed together and divided by the total of the weighted categories in use (80% as pictured above). In this example, the student has earned 53% of a possible 80% for an average of 66.25%. How that percent grade converts to a letter grade is based on the teacher's grading scale.
This is how the overall grade is achieved. Within a category, the total points possible and the total points earned are tallied together, and a percentage is calculated. If a student earns a 8/10 and a 9/12, then their category is 8+9=17 points out of a possible 10+12=22.
17/22 is 77.27%
This method of calculation accounts for the fact that one assignment is worth more than the other, as opposed to an average of averages, where each assignment is worth the same.