What describes an air mass

Acompany purchased 200 units for $ 40 each on january 31. it purchased 170 units for $ 50 each on february 28. it sold 225 units for $ 65 each from march 1 through december 31. if the company uses the lastminusin, firstminusout inventory costing method, what is the amount of cost of goods sold on the income statement for the year ending december 31? (assume that the company uses a perpetual inventory system