I'm trying to find out why it is only possible to multiply complements of percentages together and get the correct end percentage. Why can't I just multiply .20 x .30 x .15 and get the ending discount amount. We are doing trade discounts in Business Math and the fast way to get the ending amount. Any ideas. Sorry this question isn't very clear, it's hard to ask. Thanks to anyone who knows!

In math terms, given discounts A and B you are asking why the product of (1-A)*(1-B) is not the same as 1-AB; well the product of (1-A)*(1-B) is 1 - A - B + AB, which is nothng at all like 1-AB.