I heard from support yesterday. If you hold on the variable button (there should be one variable option available to you on the app keypad) a menu will pop up that will give you the option of other...

1. Közösség
2. Mobile Platform Questions and Feedback
3. Unable to Type Variables for Math Exercises

I am also having this problem on my Android app. There doesn't actually appear to be a way to pull up the keyboard while in the app, unless I'm missing something obvious? 

1. Közösség
2. Mobile Platform Questions and Feedback
3. Unable to Type Variables for Math Exercises