Calculating Values

[Dan]

I am designing an accounts receivable form and I am
having trouble with a couple of calculations. On my form
there is a subform that has all of the transaction, the
payments that is due and the payments that have been
received. I have it so that you click on the record that
is on the subform that contains the amount that is due.
You then click a button and it brings up the bill form
and inserts the values.

I am having trouble getting two values, the first and
easiest is the previous payment. This value is located in
the 'amount paid' field that is on the subform.

The other value that I need to calculate is the previous
balance. This value is the sum of all of the records,
before the selected record, in the 'amount due' field
minus all of the records in the 'amount paid' field.