I have a spreadsheet like the example below:

Cust. Id ref # Total inv paid amout Inv# Indv.
invoice amt.

james ck# 123 500 101 250
james ck # 123 500 102 100
james ck # 123 500 103 150
Mary /r123 205 134 205
joe visa234 250 101 125
joe visa234 250 102 125


What I would like to do is subtotal or add the indivial invoice amount on
the change of reference # for each customer with a formula or function. This
is going to be imported into an accounting program using Excel .csv file.