What is the equation to figure out what I need for finals?

Question

like lets say finals counts for 20 percent of my grade and i have an 81 in the class. what is the lowest grade i can get to keep the B? i know it has something to do with the numbers .20,.80, 81, and the grade you need. but what is the exact EQUATION? thanks =)

Answer 1

The equation is (0.8 * 0.81) + (x * 0.2) = 0.8   (or 81% of 80% of your grade + whatever you get for the final 20% has to equal or be greater than 80%  

Simplifying the equation,

0.648 + 0.2*x = 0.8

0.2* x = 0.152

x = 76%