Write out your equation in the form "original price = (discounted price x 100) / (100 - percentage discount)."
Use the example of a discounted price of $40 and a 20 percent discount.
Calculate (40 x 100) / (100-20) to determine what the original price was. 40 x 100 = 400 / 80 = 5 or $50.
Use the same equation and method to calculate with your factors. Example: If you bought something for $40 and it was discounted by 20%, the original price before the discount was $50.