anyone can explain it to me ?

Swanson, Inc. uses a periodic inventory system. From the following details, calculate the cost of goods sold. Beginning merchandise inventory $2,000 Ending merchandise inventory 2, 400 Purchases 20,000 Purchase Discounts 900 Purchase Returns and Allowances 1, 200 Freight In 4.400 $17, 900 $24, 300 $22, 100 $21, 900