Having a brain freeze here, and there's only one place to turn to God help me Imagine if you will, the cost of something is £10,000 and I need to add 35% to that, I get a total of £13,500. Easy. Now imagine that I know the total is £13,500 and that the percentage added to the cost is 35%, but I don't know the cost, what's the calculation to find out what the cost it? In this instance, I know its £13,500 * .7407 = £10,000 but where does the 0.7407 come from???