You wrote: pls help!! I am facing difficulty with the formula in solving this question
So the first thing that needs to be said is that you should NOT be trying to construct "the formula."
Instead, as the assignment says, "construct a spreadsheetmodel of your savings for the next 20 years." It goes on to say, "sketch the Influence diagram" --making it sound as if "influence diagrams" may be something you've been introduced to in your class. [This is a class assignment, right?]
So I'd suggest, quite seriously, forget about "a formula" for the moment. Sketch out on a piece of paper or white board, all of the factors here...create a table of what is going to add money each year, what's going to cost money each year, how you might put all that together in year 1, then year 2, then....so on.
The formula part is relatively easy. First you have to think about this in a big picture way. And THAT is what your assignment is concentrating on.