Suppose that for a particular firm the only variable input into the production process is labor and that output equals zero when no workers are hired. in addition, suppose that when the firm hires 2 workers, the total cost of production is $100. when the firm hires 3 workers, the total cost of production is $120. in addition, assume that the variable cost per unit of labor is the same regardless of the number of units of labor that are hired. what is the firm's fixed cost?