A firm reported that the total cost of producing 100 units is $20,000 and that the production of 101 units needs a total cost of $20,100. The total cost is composed of fixed and variable costs. The variable costs are equal to

$15,000, while the fixed costs are equal to $5,000. What is the marginal cost of producing the 101st unit? Suppose that these variable costs are composed of 10 workers and each worker is paid $500 per month. What is the marginal cost of the 12th worker?