by Don » Mon Dec 12, 2011 5:39 pm
Hi and welcome to the forum!
There isn't going to be a 'quick' way to do this with one simple function or formula. The best way is going to be to write it out in Excel. So, fill 20 columns in Excel with data for the interest rate and the payment for the year and then just calculate everything out using basic mathematical formulas, just like you would do by hand on a piece of paper. There isn't going to be a simple Excel shortcut to do this quickly.
If you are not sure how to do the calculations, try this link here for some help:
http://www.tvmcalcs.com/calculators/excel_tvm_functions/excel_tvm_functions_page3
Example 3.1 is closest to what you need. You can follow that example while also changing the interest rate for each period in order to get the answer.
hope this helps!