Why not just put your data in a spreadsheet and analyze it?
Maybe you don't have a spreadsheet, that would be one reason.
You can formulate general principles after you solve it.
"I thought math guys would give the best, most accurate answer."
Johnny, you are a math guy, too, unless you are a math gal.
This is arithmetic.
Or algebra if you prefer.
There is no secret formula for this kind of problem so it rarely will attract much attention from folks who want to exercise their elegant mathmatical reasoning cells.
You will do a lot better to input to a spreadsheet, honest.
As I do this sort of stuff frequently as part of my job, I don't really want to even read through it to see if it makes sense.
But experience tells me that you will probably be quite a bit better off once you see the "if/then" analysis in rows and columns.
Works for me.