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Calculate Finance Rate, Interest Amount when we have below line Fees
The equation for the payment is  This board does not support Latex (the number formatting code) so the above is an image, the code is M is the payment calculated, n is the number of months or periods to pay off, and i is the rate per period. You can see that with i appearing 3 times in this equation, it's not possible to isolate to the form i=....  so a calculator will 'guess,' and use, say, 10%. It then raises or lowers the rate until the result is within the calculator's tolerance. I've observed that unlike other calculations, when you hit the button to calculate, a noticeable time lag occurs. I hope I haven't read too much into your question, it seemed to me this was what you asked.