calculating my max. potential grade for classes

[Buta-san]

is a hobby i like to do when im bored or right after i get my tests / assignments back or even the first day of class when i receive the syllabus.

Class 1: 3 Exams = 80% 2 HWs = 20%
80 / 3 = 26.66667

Exam 1 = 100% * 26.66667 = 26.66667(%)
Exam 2 = 76% * 26.66667 = 20.26666(%)
Exam 3 = ???

Assuming I get the full 20% from the HWs and compromising to getting a A-,

90(A-) - 26.66667(%) + 20.26666(%) + 20% = 23.06667
23.06667 / 26.66667 = 87%

I need to get 87% on exam 3 to get a A-. Even if I get a 100 on exam 3, my highest grade is about 93.5 or 94% and I would have to somehow pleasure the teacher to give me a percent for the A

TL;DR i need a 87 on my next test so i dont asian fail FUCK