Do some explanation, please.

Required information [The following information applies to the questions displayed below.] 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 $32 each. F F F Required: Determine the costs assigned to the December 31 ending inventory based on the FIFO method. Total December 15