Using the Solver
The Goal Seek command is handy for problems that involve an exact target value that depends on a single unknown value. For problems that are more complex, you should use the Solver add-in. The Solver can handle problems that involve many variable cells and can help you find combinations of variables that maximize or minimize a target cell. It also specifies one or more constraints—conditions that must be met for the solution to be valid.
The Solver is an add-in. If the Solver button does not appear on the Data tab on the Ribbon, click the Microsoft Office Button, Excel Options, Add-Ins category, and then click the Go button. ...