Stackelberg Duopolists, Firm 1 and Firm 2, Face Inverse Market $P = 50 - Q$

Stackelberg duopolists, Firm 1 and Firm 2, face inverse market demand $P = 50 - Q$ . Both have marginal cost, $M C = \$ 20$ . Let firm 2, the follower, set its output according to the formula $Q _ { 2 } =$ $15 - Q _ { 1 } / 2$ . Which of the following outputs maximizes the leader's profit?

A) $Q _ { 1 } = 7.5$
B) $Q _ { 1 } = 10$
C) $Q _ { 1 } = 15$
D) $Q _ { 1 } = 20$