Can someone please explain how to do these questions? The answers are E, D, and C

The Powder Corporation sells an Xpert snowboard that is popular with snowboard enthusiasts The company uses a perpetual inventory system, and recorded the following inventory transactions during the last two months of 2016 Purchases Sales Units 112 76 Units Unit Cost $72 71 Selling Price/Unit Nov. Beginning invento 13 Purchase 29 Sale 121 $99 Dec. 3 Purchase 56 69 16 Sale 67 98 38. Using the average cost formula, the cost of goods sold for November 2016 is (a) S9,325.69 (b) $7,803.22 (c) $9,036.58 (d) $7,884.65 (e) None of the above 39. Using the average cost formula, the cost of goods sold for December 2016 is (a) S3,365.68 (b) $4,863.86 (c) S3,086.55 (d) $4,717.73 (e) None of the above 40. Using the average cost formula, the ending inventory at December 31,2016 is (a) S3,766.60 (b) S3,963.41 (c) S3,943.18 (d) S3,767.87 (e) None of the above