Chapter 3 - Section 3.1 - Quadratic Functions and Models - 3.1 Exercises - Page 253: 55

n = 30

$E(n) = \frac{2}{3} n - \frac{1}{90} n^2$ Maximum is at the vertex, which is $V = \frac{-b}{2a}$ $b = \frac{2}{3}$ $a = -\frac{1}{90}$ Thus $V = \frac {-\frac{2}{3}}{2* -\frac{1}{90}}$ $V = 30$ Thus the maximum is when n = 30