I think you're asking for a numerical answer, but you didn't mention
your score on the final.
Since the final is worth 10% (one tenth) of the grade, that 71 that you
have going into the final is worth 9/10 of the grade. So the way you
calculate your overall average is
Overall score = 1/10 of [ (71 x 9) + score on the final ] .
If you scored absolute zero on the final, your overall average would be
Overall score = (1/10) x [ (71 x 9) + 0 ] = (1/10) (639) = 63.9% .
That's the lowest overall score for the class that you can have.
The highest overall score you could have would be if you got 100% on the final.
Then
Overall = (1/10) x (639 + 100) = (1/10) x (739) = 73.9% .
If you got a 50% on the final, your overall would be
(1/10) x (639 + 50) = (1/10) x (689) = 68.9% .
In general, your overall score for the class, after the final, is going to be
63.9% plus (1 tenth) of your score on the final.
Good luck !
