We built Calculate Percentages for people that struggle with the composition of how certain percentages are calculated. I guess the best way to describe the app is it has visual formulas allowing a clearer understanding of the five different forms of calculating percentages the app offers
http://itunes.apple.com/us/app/calculate-percentages/id558127395?mt=8
http://itunes.apple.com/us/app/calculate-percentages/id558127395?mt=8