A shop can sell $x$ lamps a week at a price of $p = 120 - 2x$ rupees each. What is the marginal revenue when $x = 20$?

Explanation

1. 01

   Revenue is price times quantity: $R(x) = x(120 - 2x) = 120x - 2x^{2}$.

2. 02

   Marginal revenue $R'(x) = 120 - 4x$; at $x = 20$ it is $120 - 80 = 40$ rupees.