I've been testing out the Sales Discount Calculator app for Android.
If you're math challenged like me, it's a great app. It's really simple to use, as well. Just enter the original price of an item, and the percentage off, and tap done. It breaks it down for you and tells you what the price will be after the percentage is taken off at the register.
Best part is, it's free. So, if you don't like it, you won't feel like you've just wasted money.