The Quantity Demanded Per Month of an Item Is Related $p = \frac { 45 } { 0.04 x ^ { 2 } + 1 } , \quad 0 \leq x \leq 20$

The quantity demanded per month of an item is related to the unit price by the demand equation $p = \frac { 45 } { 0.04 x ^ { 2 } + 1 } , \quad 0 \leq x \leq 20$
where $p$ is measured in dollars and $x$ is measured in units of a thousand. How many items must be sold by the manufacturer to maximize its revenue?
Hint: Recall that the revenue is given by $R = p x$ .