Question 380117
Your grade can be calculated as:

Final grade = (current grade)*.75 + (exam grade)*.25

You said "at least a B".  I'm going to assume you mean 80% or above.

80 = (66.5)*.75 + (exam grade)*.25
80 = 49.875 + (exam grade)*.25
Subtract 49.875 on both sides
30.125 = (exam grade) * .25
Divide by .25 on both sides to get (exam grade) by itself
120.5 = exam grade

In order to get at least an 80%, you'd need 120.5% on your final exam.