Adding tax in is fairly easy - "FinalCost = BaseCost * (1+TaxRate)" is how - TaxRate is converted to decimal form. Thus, for 17.5%, you'd multiply the base cost by 1.175 to get it.
How about finding how to go from two X, Y, and Z positions instead of just X and Y? How about finding this angle (pan and tilt)?
I believe that everything in the universe is numerical in some form or another and since it is, there are mathematical rules that can be applied to pretty much anything.