1. ## average

Jerry had an average score of 85 on his first eight quizzes. He had an average score of 81 on his first nine quizzes. What score did he earn on his ninth quiz?

2. Originally Posted by nancymcwilliams
Jerry had an average score of 85 on his first eight quizzes. He had an average score of 81 on his first nine quizzes. What score did he earn on his ninth quiz?
$85 = \frac{\text{Sum}}{8} \Rightarrow (8)(85) = \text{Sum}$ .... (1)

$81 = \frac{\text{Sum} + \text{score}}{9} \Rightarrow (9)(81) - \text{score} = \text{Sum}$ .... (2)

Equate equations (1) and (2) and solve for score: (8)(85) = (9)(81) - score => ....