When you decide to publish a spreadsheet with Excel Services, you must decide which field(s) users would most likely need to manipulate. If you begin with the example of the simple interest calculation program mentioned earlier, you can quickly determine that the user only needs to vary three values:
The initial loan amount
The loan interest rate
The monthly payment
With these three pieces of information, you can calculate each month's payment, determining the portion that pays the interest due and the amount used to reduce the loan's principal amount. You can then create a running total of these two numbers to display by month the amount in interest you paid to the bank for the privilege of borrowing ...