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?
$\displaystyle 85 = \frac{\text{Sum}}{8} \Rightarrow (8)(85) = \text{Sum}$ .... (1)
$\displaystyle 81 = \frac{\text{Sum} + \text{score}}{9} \Rightarrow (9)(81) - \text{score} = \text{Sum}$ .... (2)
Equate equations (1) and (2) and solve for score: (8)(85) = (9)(81) - score => ....