To calculate the grade you need on Exam 3 to reach a 70% or higher, you can use the following formula:
(0.07 x enrichment assignments grade) + (0.16 x homework assignments grade) + (0.19 x Exam 1 grade) + (0.19 x Exam 2 grade) + (0.19 x Exam 3 grade) + (0.20 x Final exam grade) = 0.70 x total grade
Since you want to know what grade you need on Exam 3 specifically, you can rearrange the formula to isolate for Exam 3:
Exam 3 grade = (0.70 x total grade - 0.07 x enrichment assignments grade - 0.16 x homework assignments grade - 0.19 x Exam 1 grade - 0.19 x Exam 2 grade - 0.20 x Final exam grade) / 0.19
Plugging in your current grades, we get:
Exam 3 grade = (0.70 x 100 - 0.07 x 64.21 - 0.16 x ? - 0.19 x ? - 0.19 x ? - 0.20 x ?) / 0.19
Since we don't have your grades for Homework assignments, Exam 1, Exam 2, and Final exam, we'll use variables instead. Let's call them:
H = homework assignments grade
E1 = Exam 1 grade
E2 = Exam 2 grade
F = Final exam grade
Substituting these variables, we get:
Exam 3 grade = (0.70 x 100 - 0.07 x 64.21 - 0.16 x H - 0.19 x E1 - 0.19 x E2 - 0.20 x F) / 0.19
Simplifying:
Exam 3 grade = (7 - 0.44947 - 0.16H - 0.19E1 - 0.19E2 - 0.20F) / 0.19
Exam 3 grade = (6.55053 - 0.16H - 0.19E1 - 0.19E2 - 0.20F) / 0.19
Now, let's say you want to achieve a minimum grade of 70%. In other words, you want your total grade to be at least 70%. Using the same formula, we get:
(0.07 x enrichment assignments grade) + (0.16 x homework assignments grade) + (0.19 x Exam 1 grade) + (0.19 x Exam 2 grade) + (0.19 x Exam 3 grade) + (0.20 x Final exam grade) = 0.70 x 100
Simplifying:
0.07 x enrichment assignments grade + 0.16H + 0.19E1 + 0.19E2 + 0.19 x Exam 3 grade + 0.20F = 70
Substituting the expression we derived earlier for Exam 3 grade:
0.07 x enrichment assignments grade + 0.16H + 0.19E1 + 0.19E2 + (6.55053 - 0.16H - 0.19E1 - 0.19E2 - 0.20F) / 0.19 + 0.20F = 70
Simplifying:
0.07 x enrichment assignments grade + 1.01005 - 1.05263H - 1.01053E1 - 1