I need to develop a "current inventory" spreadsheet from two existing
spreadsheets; one tracks add and the other tracks substractions/sold. Each
spreadsheet tracks product, size and quantity. Here's an example:

Product Size Quantity
Blue Pants Lrg 5
Red pants Sm 5
Blue Pants Med 6
Blue Pants Lrg 7

Information is entered into these spreadsheets as events happen. So
multiple similar product/size combinations exist in both spreadsheets. My
challenge is adding up all the matching combinations in the two spreadsheets
and then substracting the solds from the add. Also need to consider that
some product/size combinations have never sold - so these combinations would
not appear on the sold spreadsheet.

Thanks in advance for any suggestions.