I have a 74.4 C and i get a B on midterm what will my grade be?

I have a 74.4 C at my school its a C, If i get a B or higher lets say i get a 85 B os somthing what would my grade be?? The final grade what would it take to make that 74.4 go to a 80 B ????? Points will be awarded to best anwer My midterm is worth 15%???