Weighted averages allow one value to be worth more than another value. The most common time you will see weighted averages with percentages is dealing with grades. If your professor is weighing you grades, then he will provide a syllabus that should list the weight for each grade you have. Once you find out your percentage grade on each test, then you can apply the weights to the grade and calculate your weighted grade.
Find your percentages on each assignment. For example, assume you received 70 per cent, 90 per cent and 85 per cent on your class's three assignments.
Find the weight for each assignment on the class syllabus. In the example, assume the weights for each grade are 50 per cent, 30 per cent and 20 per cent, respectively.
Multiply your percentage by the weight. In the example, 50 per cent times 70 per cent equals 0.35, 30 per cent times 90 per cent equals 0.27 and 20 per cent times 85 per cent equals 0.17. These are your weighted grades.
Add together your weighted grades and multiply the result by 100. In the example, 0.35 plus 0.27 plus 0.17 equals 0.79, then 0.79 times 100 equals 79 per cent.
This method works with any type of percentages that you weigh, not only grades.