Thanks Chris. Now this is where it gets tricky.
A product has a Yield an opening stock, Purchases, Credits & closing stock all of which are expressed as afore mentioned.
Yield = PackSize (6,8,10,12,15,24) in this example i'll use 15
OS = 3.13 - 3x15 + 13 = 58
Pur = 6 - 6x15 = 90
Less any...