Excel - Compute Third Field Given Two Others

I know how to do this programmatically but I wonder if I can do this using only formulas... it's not obvious to me.

Referring to the image below, I want to be able to calculate the field with no entry when the other two are filled in.
 
The overall formula is Cap Rate = Income / Price... so if someone enters income and price, I'll compute the cap rate, if they enter cap rate and price, I'll compute compute income and so on.
 
At this point, the only way I can see to do it is to have a second set of fields below the first that contain the formulas.  Any other thoughts or am I going to have to add a "Compute" button and a chunk of VBA code?

TIA