As you will note in your other question, some of the Experts showed you PHP implementations using IRR function. The internal rate of return usually goes with NPV (net present value).

http://en.wikipedia.org/wiki/Net_present_value#Formula

Therefore, you are seeking the present value of the advance (A) here. In other words, the present value of the advance equals the present value of the annuity payment at a specific rate of return. If you do not use the correct interest rate, these values will not equal.

http://en.wikipedia.org/wi

Therefore, you are seeking the present value of the advance (A) here. In other words, the present value of the advance equals the present value of the annuity payment at a specific rate of return. If you do not use the correct interest rate, these values will not equal.