When an item goes on sale, or rises in price, this is often referred to as a percent decrease or increase. This isn't anything new, no new processes to learn or rules to memorize. Basically, it's just turning a fraction into a percent.
To find percents of increase or decrease, all we need to do is take the difference in amount and divide it by the original amount, just as you would to turn a fraction into a decimal. Finally multiply this by 100, just as you would to turn a decimal into a percent.