help with functions/stats/trig question?

Question

Mr. Field counts chapter tests as 40% of a student's grade, homework as 20%, quizzes and class work as 20%, and final exams as 20%. Lydia has an average of 89 before the final. What score must she get correct on the final to bring her grade up to a 90?

Answer 1

Since we know that her average before the final is 89, and that is 80% of her final grade, we can make an equation letting x be her grade on the final exam.

So .80(89) + .20 x = 90

Then 71.2 + .2 x = 90

Subtracting 71.2 from both sides gives

.2x = 18.8.

Then dividing by .2 gives

x = 94

She must get a 94 on the final exam to bring her score up to a 90.