First, the socially optimal quantity of paper is found by setting MC
p + MC
e = p or
. Rearranging yields Q = 30. The competitive equilibrium is found by setting MC
p = p or 10 + Q = 100 - Q. Rearranging yields Q = 45. The deadweight loss of those additional units equals (45 - 30) ∗ (70-40)/2 = 225. Under monopoly, the firm sets
. The monopoly produces the socially optimal quantity, and therefore has no deadweight loss. Social welfare is greater under monopoly.