Counting what's left in a list

[Guest]

I have a list of a bunch of different products and their product numbers,
within the list several products are grouped together (as in a department) I
have a Vlookup table to let me enter the numbers of the products I already
have purchased, but I would like to be able to have a cell tell me how many
are left to purchase in a given department. For example I enter all the
product numbers for things I have purchased already, but I want to know how
many products (NOT which products just how many) are left to purchase in a
department. In my list I have given each department a range name, but not
sure if that helps, or how to use that. Any help would be appreciated.

 

[Guest]

try
=counta(all products list)-counta(product bought list)

 

[Guest]

That didn't quite work, it only gave me the difference of how many items are
in the products list and the number of items in the purchased list instead of
saying how many from a department were left. Would a range name help?