This is often done using the goal seek function in Microsoft Excel or what-if analysis via computer programs. Goal-seek software only works if there’s only one input value, however.

Susan Harkins shows you how to use Excel's Goal Seek what-if analysis tool to achieve ... (That’s the cell that contains the PMT() function.) The To value should equal your goal.

Enter Goal Seek and the Solver add-in, two tools that let Excel do the heavy lifting when it comes to finding those unknowns. When you have a formula in Excel but need to find the value of one ...