Kirsten owns a business on a street on which the street lights have been inadequate and need to be updated. She learns that each light costs $20. She has two neighbors that would each benefit from improved street lights. A reliable estimate of their willingness to pay is P = 20 - Q. Kirsten's own demand is represented by the equation, P = 40 - 2Q.

1. Derive the aggregate willingness to pay curve. What is the socially optimal number of street lights for this street?
2. If the private market were to provide the street lights, how many would be installed?
3. Describe the free-rider and inefficiency situations in this case.