Consider a market of pens where there are 15 firms: one firm sells 870 pens, 4 firms sell 250 pens each, and 10 firms sell 120 pens each. a. Determine the market shares for each firm. b. Calculate the four-firm concentration ratio for this industry. c. Calculate the eight-firm concentration ratio for this industry. d. Calculate the Herfindahl-Hirshman index for this industry. e. What are the measures above used for?