Question 932359
Your final is worth 15% of your grade. So 100-15 = 85% of your grade is for the remainder of the assignments and other stuff.



Let x = score on the final (score in decimal form)



0.83*0.85 + 0.15x = 0.90



0.7055 + 0.15x = 0.90



0.15x = 0.90-0.7055



0.15x = 0.1945



x = 0.1945/0.15



x = 1.2966666



Multiply by 100 to get 100*1.2966666 = 129.66666%. It is not possible to get a score of 129.67% on the final. 



So it is not possible to go from an 83% overall grade to a 90% overall grade.



If you got 100% on the final, then 0.83*0.85 + 0.15*1 = 0.8555 so the best you can get is 85.56%



Note: the teacher may drop lowest scores or add more extra credit in some way, so it's always best to ask the teacher. My answer gives you a ballpark idea but the teacher will know better than me (since it's their own personal grading system, they can do whatever they want).