How to calculate how much my final exam will affect my final grade in the class

I have a 90% in the class
The exam is worth 20%
How do I calculate what my final grade in the class will be depending on my exam mark

(I'd like if you could show me an example, suppose I got 50% on the exam)

so as of right now you have 90% of 80.
100-20=80
90% of 80=72
if you get a zero on your final that will be your final grade.
if you get a 50% then your grade will be 82
.5*20=10
72+10=82
If you get a 100 in your final your grade will be a 92
72+20=92

http://www.conquercollege.com/gradecalc/ use this