Upon occasion I have the need...THE NEED FOR SPEED!...oops, sorry got carried away Seriously, I'll come across a formula (like right now I'm researching turbos) and need to solve for one of the variables, typically the one that's on the wrong side of the equal sign. Excel does have a function, Goal Seek, that allows one to solve for an unknown but it's not necessarily easy to use. What I did was put a 'wrapper' around it to make it much more user friendly. I based it on a similar application I use on my PDA but my application doesn't require registration -- it's 100% FREE.
Disclaimer: This isn't something a 3 year-old could use right away. It's meant for people who have some education about problem solving and unknown variables. If you've graduated high school (or are about to) then you've got enough education The instructions should provide adequate direction along with the workbook.
Instructions:
http://www.icbm.org/erkson/personal/Better_Goal_Seeker.doc
Sample workbook:
http://www.icbm.org/erkson/personal/Better_Goal_Seeker.xls
BTW, the examples in the workbook can be deleted without affecting anything. Formulas can be added and deleted at will. It's just your standard, ordinary Excel workbook. The only thing different about it is that there's a macro in it to help one solve for an unknown variable within an equation. As you can see, you can have multiple formulas on a sheet, even on other sheets. Enjoy!