formula crossing multiple worksheets

[Garnet]

This is what I am trying to do. I have a master inventory sheet that I would
like the item amounts to change depending upon what items are shipped or
received.

The master inventory list (Inventory) has the product numbers (item #) in
column B and the number of items (#) in column I. On the received sheet
(Received) the product numbers (item numbers) is in column A of the received
sheet, and the number of items (#) in column J. On the shipped sheet
(Shipped) the product numbers are in column A (item number) and the number of
items (#) is in column J.

I know this can be done, but I just cannot seem to figure out how to make
the master inventory data change according to what is entered in the shipped
or received sheets. The products listed on the master inventory sheet are
static, however the shipped and received sheets will be data entered as they
occur. The product numbers (item numbers) are what I was trying to use as a
key.

Also, these sheets will be used monthly. At the beginning of each month,
how do I transfer all the data from the master inventory to the new months
workbook without having to re-enter it all again?

Thanks lots,
Garnet