explain- For public goods, we don't add quantities horizontally; we add prices vertically. Since hamburgers are now nonrival, Bill and Ted each consume all of them, so the social marginal benefit is the sum of their separate marginal benefits. For Bill, Q = 21 6P where P is his marginal benefit; solving for P yields P = 3.5 Q/6. For Ted, Q = 6 3P, so P = 2 Q/3. The total social marginal benefit is therefore P = 5.5 Q/2