Goal Seek is part of a suite of commands sometimes called what-if analysis (what-if analysis: A process of changing the values in cells to see how those changes affect the outcome of formulas on the worksheet. For example, varying the interest rate that is used in an amortization table to determine the amount of the payments.) tools. When you know the desired result of a single formula (formula: A sequence of values, cell references, names, functions, or operators in a cell that together produce a new value. A formula always begins with an equal sign (=).) but not the input value the formula needs to determine the result, you can use the Goal Seek feature available by clicking Goal Seek on the Tools menu. When goal seeking (goal seek: A method to find a specific value for a cell by adjusting the value of one other cell. When goal seeking, Excel varies the value in a cell that you specify until a formula that's dependent on that cell returns the result you want.), Microsoft Excel varies the value in one specific cell until a formula that's dependent on that cell returns the result you want.
For example, use Goal Seek to change the interest rate in cell B3 incrementally until the payment value in B4 equals $900.00.