Answer: 11 cars
Step-by-step explanation:
Socially optimal quantity is determined where,
Demand = Marginal Social Cost
We have the demand function, now we must calculate the MSC.
We can do that as,
MSC = Marginal Private Cost + Marginal External Cost
MSC = 20 + 5q + 10
MSC = 30 + 5q
Going back to our original formula we can now solve for the socially optimal quantity as,
Demand = MSC.
100 - 5q = 20 + 2q
80 = 7q
q = 11.42
q = 11
11 is the socially optimal number of cars produced.