Suppose that a firm has only one variable input, labor, and firm output is zero when labor is zero. when the firm hires 6 workers the firm produces 90 units of output. fixed costs of production are $6 and the variable cost per unit of labor is $10. the marginal product of the seventh unit of labor is 4. given this information, what is the marginal cost of production when the firm hires the 7th worker?