If you want to practice mental math fractions and percentages for your case interviews, I've released an app called 'Case Math' on the iOS store to do just that. https://apps.apple.com/us/app/case-math/id1507653375?mt=8&ign-mpt=uo%3D4
I've designed it to focus only on fractions for three key reasons:
1) Fractions are involved in many calcs: market sizing, margins, market share etc.
2) Fractions are typically harder and less practiced than other operations - addition, subtraction & multiplication
3) I couldn't find other mental math apps that allowed practice using non-exact answers. E.g. in a case you'd typically be allowed to estimate 6/17 to within 5-10%
I've found it helpful to incorporate into my daily drills and thought others might as well. If you give this app a try, I'd love to hear your thoughts on it - if you think it's useful, and what other features you think would help improve it.