MentalMathX is a simple Android game which tries to focus on speed and more difficult exercises than other mental arithmetic trainer apps. As a bonus it's ad and spy free and you even get the source code. Isn't that great?
As it's a hassle to setup Android Studio from a project made with an older or even a newer version of AS, there's just a simple advise: try to import it. If that doesn't work you should look into the Known Issues of AS. If that still doesn't work, feel free to contact me.
This is Free Software (GPLv3) but please don't clutter it with ads. That would be a dick thing to do.
The app icon is taken from Freepik and is published under CC BY 3.0. It was downloaded from flaticon GPLv3 does not apply to the icon!