text: You have 87% on 85% of your grade.  Let x be the grade you need to get on the final to get a B\n\n0.85*87 + 0.15*x = 80\n73.95 + 0.15x = 80\n0.15x = 6.05\nx = 6.05/0.15 = 40.3\n\nSo, you would nee to get above a 40% on the final to stay above 80%.