A company purchased 400 units for $20 each on January 31. It purchased 200 units for $30 each on February 28. It sold a total of 270 units for $90 each from March 1 through December 31. If the company uses the last-in, first-out inventory costing method, calculate the amount of ending inventory on December 31. (Assume that the company uses a perpetual inventory system.)

a. $23,100 b. $6,600 c. $9,900 d. $330

please show me how to do this