The table above shows the marginal benefit from providing police protection in a community of two people, Jake and Elwood. Police protection is a public good

a) What is the marginal social benefit to community from the 4th police car on duty?
b) If the marginal social cost of a police car on duty is $37, what is the efficient number of cars on duty?
c) If the marginal social cost of a police car on duty is $70, what is the efficient number of cars on duty?

a) The community's marginal social benefit is the sum of Jake's marginal benefit and Elwood's marginal benefit. So the marginal social benefit from the 4th police car is $15 + $8 = $23.
b) The number of cars on duty is efficient if the community's marginal social benefit equals the marginal social cost. The marginal social benefit is the sum of Jake's marginal benefit and Elwood's marginal benefit. When three cars are on duty, the marginal social benefit is $25 + $12 = $37, which is the same as the marginal social cost. So the efficient number of cars is 3.
c) As before, the number of cars on duty is efficient if the marginal social benefit equals the marginal social cost. And also as before, the marginal social benefit is the sum of Jake's marginal benefit and Elwood's marginal benefit. But in this case, because for any number of police cars on duty marginal social cost exceeds marginal social benefit, the efficient number