Suppose that the market for cigarettes is perfectly competitive and given by the following private demand and private supply functions (with P being the price of a carton of cigarettes and Q being the number of cartons per year):

P = 100 - 10Qd

P = 10Qs

-What is the amount of per carton Pigouvian tax that needs to be levied in order to bring this market into a socially efficient equilibrium?