What is the four-firm concentration ratio (CR4)? What is the Herfindahl-Hirschman Index (HHI)? How do these two measurements differ? How are they similar? Calculate the CR4 and the HHI for the following two situations: First, a duopoly with equal market share. Second, a duopoly where one firm has 80% share, and the other has 20% share. Explain any noticeable differences between the CR4 and HHI results for these two situations.