We add the demands of private goods horizontally but add the demands of public goods vertically when determining the associated marginal benefit to society. Why do we do this, and why are the procedures different for public and private goods?

Suppose that each country completely specializes in the production of the good in which it has a comparative advantage, producing only that good. in this case, the country that produces jeans will produce million pairs per week, and the country that produces corn will produce million bushels per week.