Donna's Marbles generated $73,500 in total sales for the month of September. Her total variable costs were just $52,500, leaving her with total contribution margin of $21,000. The selling price per unit of marbles (a small bag is considered a unit) is $7. How many units did Donna sell that month? Number of units sold What was her variable cost per unit? Variable costs $ What was her contribution margin per unit? Contribution margin $ units per unit per unit