adding up totals of various documents or sheets

this problem must have already been discussed on this board, but i couldn't find any thread that helped me. i'm also a numbers newbie - so please excuse me, if this quesion is stupid.


i've made an invoice document, simlilar to the invoice template that comes with numbers 08.

i duplicate an old invoice to write a new one (i could also just duplicate the sheet, so all invoices are in the same document, if that makes it easier).

now i want a document (or a sheet within an document that contains all invoices) that lists all the sums of the invoices and adds them up. i want a total of all the invoices i create, a total that *automatically updates* when i create a new invoice by duplicating an older invoice.

thanks.