may I have an explanation so I can understand?

Concord Company's record of transactions concerning part WA6 for the month of September was as follows. Purchases Sales September 1 (balance on hand) 300 @ $ 11.40 September 4 400 3 200 @ 11.50 17 600 12 300 @ 11.65 27 300 16 300 @ 11.70 30 200 22 500 @ 11.70 26 300 @ 11.80 (a1) Calculate average-cost per unit. Assume that perpetual inventory records are kept in units only. (Round answer to 2 decimal places, e.g. 2.76.) Average-cost per unit $