Android app that helps student master mental calculation

Dear community,

I'm a former teacher of math from Russia and a developer. Not long ago I launched an app that will help students master mental calculation. You can see the app description and download it here: https://goo.gl/EdLbtE.

I'd like to ask for any feedback on the app: what do you think of it? What features should be added so that teachers and students would  use it? Please feel free to make comments or ask me any questions.

Sincerely,

Anton Tkachenko