Recently I started work on an iPhone app and came across a small problem. As part of the application I needed to know how much tax was added to a value when I only knew the tax percentage and the total after tax. Now if the tax rate was a fixed amount a formula would have been easy to generate (for example the UK has 20% tax so to find the tax paid you simply divide the total by 12 and multiply this by 2), The problem I had is that the tax rate was unknown as it was to be a user input.

I asked a few people and found that nobody I spoke too really knew any way to find this out. Most people suggested just working out the percentage of the total amount however this of course will not work.

Here is how I eventually worked this out : Continue reading “Calculating How Much Tax Was Added When Only Knowing The Percentage.”