How do I calculate the following microeconomics problem?

A firm is producing 20 units with an average total cost of $25 and a marginal cost of $15. It increases production to 21 units, which of the following must occur?

• Marginal cost will decrease
• Marginal cost will increase
• Average total cost will decrease
• Average total cost will increase

Marginal cost = change in total cost / change in quantity

Average total cost = total cost / quantity

Marginal Cost = $15

Quantity = 20

Average total cost = $25

Total Cost = Average total cost X quantity = ($25x20) = $500