Annuity with changing annual amount

[accProgrammer]

I am working on a program for a client. It calculates the present value of an annuity. That part I have completed. The problem is he wants to be able to change the annual some after x amount years. The new annual sum will have accrued compounded interest. He then needs this annual sum to start after the original number of years. For example if the initial sum is 20000 with an interest rate of .08. The first annual sum is 25000 for 10 years. The present value is 187,752.03. After that 10 years he wants to change the annual sum to 15000 for 10 years(after the initial 10 years). This 15000 has been compounding interest. I need the present value of this. What all do I need to calculate to make this happen successfully. All of my figures seem too high.

 

[kirby]

Suggest you model this in Excel first then build a program from that experience later. Build a list of expected cash flows then use Excel's NPV (actually a misnomer cause it is for PV not NPV) to get the present value.