I agree with other coaches. When we talk about mental math, I can also advise you on an app called 'Case Math' on the iOS store to practice mental math fractions and percentages for your case interviews (https://appshttps://apps.apple.com/us/app/case-math/id1507653375?mt=8&ign-mpt=uo%3D4.apple.com/us/app/case-math/id1507653375?mt=8&ign-mpt=uo%3D4)
There are basically two reasons to do that:
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
If you need some help with your application, just drop me a line.