Could someone explain how to solve this problem. Understanding average cost per unit is something I struggle with. Thanks!

Problem 6-09A al-a2, b (Part Level Submission) Cullumber Co. began operations on July 1. It uses a perpetual inventory system. During July, the company had the following purchases and sales. Date Units Purchases Unit Cost $130 Sales Units $144 July 1 July 6 July 11 July 14 July 21 July 27 $155 (al) Calculate the average cost per unit at July 1, 6, 11, 14, 21 & 27. Average cost for each unit July 1 4 July 6 4 July 11 4 July 14 4 July 21 4 July 27 4