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 sources made with an older or newer version of AS this just states: try to import it. If it doesn't you should look into the Known Issues of AS and if then still doesn't work contact me.
This is Free Software (GPLv3) but please don't clutter it with ads. That would be a dick thing to do.