Goal Seek in Excel
Get the result you want by adjusting a value using Goal Seek.
If you know the result that you want from a formula, but not the input value the formula needs to get that result, you can use the Goal Seek feature. For example, use Goal Seek to change the marks in cell B5 until the value in B6 equals 80.
On the Data tab, in the Data Tools group, click What-If Analysis, and then click Goal Seek.
In the Set cell box, enter the reference for the cell that contains the formula you want to resolve. In the example, this is cell B6.
In the To value box, type the result you want. In the example, this is 80.
In the By changing cell box, enter the reference for the cell that contains the value you want to adjust. In the example, this is cell B5.
Note: This cell must be referenced by the formula in the cell you specified in the Set cell box.
Where is Goal Seek in Microsoft Excel 2007, 2010 and 2013