The weekly supply and demand for packs of cigarettes in the U.S is given in Figure 18P-1.

a. Suppose cigarette smoking causes a $6/pack external cost on nonsmokers. Draw the demand curve that would exist if the externality associated with smoking were internalized by consumers.
b. How many packs of cigarettes are consumed per week?
c. What is the efficient number of cigarette packs?
d. What would be the total gain in surplus if this externality could be internalized?