Take your current average in that class. If the Final Exam is worth 20%, then every point above your current average will raise your grade .2 points, and every point below your current average will lower your grade .2 points.
For example, you have a 70% average...that means that 80% of your final grade is 70%. Looking at this in fraction form, 80% = 80/100 = 4/5. 4 parts of your grade is 70, for a total of 280. The final part of your grade will be the exam. If you get a 71 on the final, then your average would be 351/5 = 70.2%