If you want to use Excel's Goal Seek on multiple cells simultaneously, you'll need a much more powerful tool. Fortunately, one such tool comes with Excel. Let's have a look at the Solver add-in.

With this example, type "A1." Click "Solve." Excel will replace cell A1 with the equation's solution. With this example, it will insert the value "21," the equation's solution, into cell A1.

Check the "Solver" check box in the new window that appears, then close all dialog boxes. Excel adds the command Solver to its Data tab. Click "Solver," then click your objective formula's cell.