Final answer:
To calculate marginal cost, divide the change in total cost by the change in quantity. However, the provided cost table is incomplete and does not provide the necessary information to calculate the specific marginal costs for outputs of 40, 80, and 120.
Step-by-step explanation:
Marginal cost is the additional cost of producing one more unit of output. To calculate the marginal cost, we take the change in total cost and divide it by the change in quantity. For example, if the output increases from 40 to 60 units and the total costs rise by $80, then the marginal cost for each additional 20 units will be $80/20, or $4 per unit.
To find the marginal cost when output is 40, 80, and 120, we need the corresponding change in total cost for each output level. However, the cost table provided in the question is incomplete and does not include the necessary information. Therefore, we cannot determine the specific marginal costs without the missing data.