How do you create three-way formulas in Pan X?

For example, if the formula is:

- Cost of Sales = Volume (units) x Unit Cost, e.g. $100 = 50 units x $2.00/unit;

how can I enter a new value in the Cost of Sales field (the calculated field) to calculate 2) a new volume figure, given that the Unit Cost figure is unchanged, or, 3)

how can I calculate a new Unit Cost figure, given that the Volume figure is unchanged?