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?

Please also provide the working out and necessary formulas.