text: how do i calculate  the lowest grade i can get on the final to maintain my grade.? i have an 87 in a subject. it cant move to an a- (dont ask why), so what is the lowest grade i can get on the final in order to just keep an b and get at the lowest an 80%.   final is worth 15% of grade.  thanks! 10 points. you have 87% on 85% of your grade.  let x be the grade you need to get on the final to get a b  0.85*87 + 0.15*x = 80 73.95 + 0.15x = 80 0.15x = 6.05 x = 6.05/0.15 = 40.3  so, you would nee to get above a 40% on the final to stay above 80%.