The easiest way to take 20 percent off a price is to multiply the original price by 0.8. This will give you the price after the 20 percent discount has been applied. For example, if the original price is 80, which is the price after the 20 percent discount.

CR shopping editor Samantha Gordon likes to use Honey, a free tool that you can download on any browser. Once installed, it will automatically seek out discount codes and apply them when you're purchasing a product online. "It's a huge time-saver," she says.