The student has earned the following scores for each category: tests-83, final exam-75, quizzes-90, homework-100. We need to calculate the student's overall grade. To calculate a weighted average with percentages, each category value must first be multiplied by its percentage.Then all of these new values must be added together. In this example, we must multiply the student's average on all tests (83) by the % that the tests are worth toward the final grade (40%).Please note that all %'s must be converted to decimals before you multiply. Similarly, the final exam score (75) will be multiplied by its % (25).he student has earned the following scores for each category: tests-83, final exam-75, quizzes-90, homework-100. We need to calculate the student's overall grade. To calculate a weighted average with percentages, each category value must first be multiplied by its percentage.