Stanley's Bicycles store buys bicycles on average for $600 and sells them on average for $750. He pays a sales commission of 15% of sales revenue to his sales staff. Stanley pays $1,400 a month rent for his store, and also pays $3,000 a month to his staff in addition to the commissions. Stanley sold 120 bicycles in June. If Stanley prepares a contribution margin income statement for the month of June, the contribution margin would be $4,500. How do you get this answer ($4,500)?