Mason Corp. has the following information for 2020: Item Amount Sales $180,000 Beginning inventory 64,800 Freight-out 16,200 Purchases 77,400 Sales commissions 9,000 Purchase discount 8,000 Purchase returns 5,400 Freight-in 7,200 Cost of goods sold 108,000 What is Masons gross margin for 2020 under a periodic inventory system?