If you want to practice mental math fractions and percentages for your MBB / consulting case interviews, my mate and I released an app on the iOS store called 'Case Math' https://apps.apple.com/us/app/case-math/id1507653375?mt=8&ign-mpt=uo%3D4.
It's 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) We 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%
We've found it helpful to incorporate into our 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.