| use your rows for each month, and have a column for outstanding debt, and a column for amount paid. Enter data for amount paid each month, your formula for each month past the first is (Amount_owed_last_month - Amount_paid_last_month)*1.05.
Does this help?
I made a quick spreadsheet for you if you want it just ask, I can't put it up from here but I can do so from home. |