As for the app - great idea and nice implementation, but for someone who understands what to do right from the beginning, "hints"-popups are really annoying. Maybe add a button "I know what I'm doing" or something like that to allow to skip them?
Are you thinking of making more math-related games?
- Programmed in Java, using libgdx as the engine
- Sound effects made with sfxr
- Art made with Gimp and Inkscape