Answer:
marginal product of labor = 5 widgets per hour
Step-by-step explanation:
In order to maximize profits, the firm must produce the output quantity where marginal revenue = marginal cost. In this case, the marginal revenue is $2, so the marginal cost must also be $2.
If hiring the last widget maker costs $10 per hour, and the marginal cost per widget is $2, then the worker must be able to produce 5 widgets.