Teachers often use weighted percentages to assign significance to different assignments. For example, a teacher might place more emphasis on students doing well on the final than on the two other tests during the class. If your class uses a weighted percentage system, you need to know the value of each of the assignments and how well you did on each assignment to figure out your grade for the class.
TL;DR (Too Long; Didn't Read)
To find your weighted average grade, multiply each grade by its assigned weight (expressed as a decimal). If the assigned weights add up to 1, you're done. If the assigned weights don't equal 1, divide your sum by the total of the assigned weights.
Convert Percentages to Decimals
Weight the Grades
Total the Weighted Grades
This process assumes that the assigned weights add up to 1 (which should be the case if you've completed all your assignments for the class term). If the total of the assigned weights isn't 1 – for example, if you haven't completed all the assignments yet – you also need to divide the result of Step 3 by the total of assigned weights (still expressed as a decimal). So if the sum of your weighted grades was 72 but the assigned weights for the assignments you've completed only adds up to .8, you'd have 72 ÷ .8 = 90 as your weighted average at that moment in time.
Divide each weight expressed as a percentage by 100 to convert to a decimal. For example, if your first test is 20 percent of your grade, divide 20 by 100 to get 0.2. If your second test is worth 30 percent and your final test is worth 50 percent, divide 30 and 50 by 100 to get 0.3 and 0.5.
Multiply each grade by its weighted percentage. If you scored 95 percent on your first test, multiply 95 by 0.2 to get 19. If you scored 80 on your second test and an 88 on your final test, multiply 80 by 0.3 and 88 by 0.5 to get 24 and 44.
Add the results from step 2 to find the weighted average. Here, add 19 + 24 + 44 to find your average to be 87.