Trey Monson starts a merchandising business on December 1 and enters into the following three inventory purchases. Monson uses a perpetual inventory system. Also, on December 15, Monson sells 15 units for $27 each. Purchases on December 7 Purchases on December 14 Purchases on December 21 10 units $13.00 cost 20 units $19.00 cost 15 units @$21.00 cost Required: Determine the costs assigned to the December 31 ending inventory based on the FIFO method.