Math people, lend me your skills

Jimmy King

I've got to do some work that includes figuring out the pre-VAT cost of an item given the cost of the item including VAT. So, the item in particular is going to be 3.50GBP. With 17.5% VAT, that means pre-VAT cost was about 2.98GBP. Rather than hard coding this 2.98GBP in my app, though, I would much prefer to take that 3.50GBP that I am given as input and determine the pre-VAT value, that way if the 3.50GBP changes at some point in the future (most likely with no one at the company bothering to tell me), I don't have to go back and make changes to the program/configuration to deal with it, it'll just figure out the new pre-VAT value automagically using them fancy math skills I never really developed.

So, basically, I need to be able to solve:
y + (y * .175) = 3.50

Atlus Parker

totalcost * .851 = pretaxcost

Give or take

Jimmy King

ah hah. I knew it was going to be something simple like that. Overcomplicating simple things is a common problem for me.

Thanks man, I appreciate it.

Vrtra Theory

I don't know if you're worried about the VAT you need to compute with ever changing, but you might want to treat both as inputs, like so:

vat = 0.175

pretaxcost = totalcost / (1 + vat)

Jimmy King

Thanks vrtra, that does work even better. I was trying to figure out (and getting close to giving up and asking) how the .851 was determined so that I could sort it out later if a different VAT was needed since that is a possibility a month or two down the line, I believe.

I should really go back to school and take some math classes. I'm one of those developers that mostly does non-math related stuff and so my already lacking math skills have slipped a lot over the years.