Level 15

Manage Customers and Income

did you read what I said
average cost per purchase date
you buy 13 for $9 on one day the cost per item is 9/13 = 0.69230 each

you buy 12 for $11 a different day, the cost per item for THAT day, 11/12=0.91666

Sell 20 and COGS shows
13 * 0.69230= 8.9999
7*0.91666=6.42
total of 15.41