Simple calculations using percent are easily done and understood by using this app, that not only shows the answer but also what computation brings to that answer. The formulas involved are displayed very systematically as well as how they can be derived. Just choose from a systematic list what problem is at hand, and go from there to a second page where the rest happens. For the sake of extreme systematics, and thus greatly reducing the number of different cases, You will use a switch to flip between input of increase and decrease when it comes to percentages.