Hi Patricia,
If you had all 5 grades and the average was 85 then the sum of the 5 grades divided by 5 would be 85. Thus the sum of the 5 grades would be 5 85.
Penny
Hi Patricia.
I'll show you how to do this with different numbers:
Let's say your quiz grades are 75, 85, 95 and want to achieve an 80 average overall. What minimum score do you need on the fourth and final quiz?
Let x be the score of the final quiz. Then the average for the four quizzes is:
(75 + 85 + 95 + x ) / 4
and we want a minimum average of 80, so:
(75 + 85 + 95 + x ) / 4 ≥ 80
Solve for x:
(75 + 85 + 95 + x ) ≥ 80 4
x ≥ 80 4  75  85  95
x ≥ 65
So for this question, you would need at least a 64 on the final quiz
to achieve an overall average of 80.
Use this method to solve your own question.
Stephen La Rocque.
