In the short run, a firm in a monopolistically competitive industry has a cost function equal to C(Q) = 300 + 5Q where Q is annual output, so the firm's annual fixed cost is 300 and its marginal cost is 5. Each firm in the industry has its own demand equal to Q_D = 100 4P, so its inverse demand is P = 25 0.25Q_D, and its revenue is Q_D(25 0.25Q_D), and its marginal revenue is 25 0.5Q_D.

(b) Calculate the elasticity of demand at the monopoly price and quantity