If you have an 82 in your class and your final is worth 20% of the grade, you will have to get a 22 on your final to have a 70 in the class as a final grade. If your final is worth 20%, the rest of your grade is worth 80%. 80% of 82 is 65.6. 70 - 65.6 = 4.4 So, 20% of the grade on your final needs to equal 4.4.
You would need at least 22% on the exam. Here is the math if the score 82 of 100 is 80% of the grade and you received a 22% on the exam worth 20%: 82+82+82+82+22 divided by five = 70. Keep in mind sometime you need a passing grade on the final exam in some classes regardless of the final average.