The price at which $x$ units sell is $p = 90 - 3x$ rupees. At what price is the total revenue greatest?

Explanation

1. 01

   $R = 90x - 3x^{2}$, and $R' = 90 - 6x = 0$ at $x = 15$ units.

2. 02

   The price then is $90 - 3 \times 15 = 45$ rupees.