My final is worth 20% of my grade if I get a 70 on my final I get an 80 grade.What if i get a 60

Let x be the rest of your average.

x(80%) + 70(20%) = 80 ==> x = (80 - 0.2(70))/0.8 = 82.5.

If you get a 60, you'll have

82.5(80%) + 60(20%) = 78.

That is a final grade of 78.

My final is worth 20% of my grade if I get a 70 on my final I get an 80 grade.What if i get a 60?

==> if you get 60, you will get 78 grade...

this is not a math class, is it ?

78.......with the 70% you will get 800/1000 , but 60% means 20 less points--> 780 / 1000