3. The demand for electric vehicles is given by =100 , where is cars in a community, while marginal private cost is =. By reducing demand for gasoline, electric vehicles generate a marginal external benefit = a. What is the equilibrium number of electric cars in the private market? b. If the socially efficient number of electric cars is =20, what is the value of ? c. If the government subsidizes electric cars by $ each, what is the value of that induces the socially - efficient outcome?