Gerri is considering purchasing up to 4 machines to operate in her factory at a cost of $200 each. If she buys 1 machine, she expects a total profit increase of $10 per year. If she buys 2 EC202-730, MSU Fall 2023 4 machines, she expects a total profit increase of $16 per year. If she buys 3 machines, she expects a total profit increase of $19 per year. If she buys 4 machines, she expects a total profit increase of $21 per year. The interest rate is 2%. Assuming the marginal opportunity cost includes only interest, how many machines should Gerri buy