The best way to develop math problem solving is to download all available case books and go just through the math problem. There is a limited number of types of problems in cases and you can cover almost all of them
In addition, you have to develop 3 calculation skills:
1) Learn how to multiply double-digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo).
2) Learn how to work with zeros. The best way - always use 10^power instead of zeros
300x9000 = 3*10ˆ2 x 9*10ˆ3=3x9*10ˆ(2+3)=27*10ˆ5
Handwritten it looks not that complicated. If you get used to writing all the numbers that way, you will never lose zeros and all multiplications/divisions will be replaced with + or -.
3) Learn the division table up to 1/11 (i.e. 5/6 = 83.3%). It will help you calculate any percentage problems
Use math tools to train (Preplounge tool, Mimir math for iOS, Math tool on Viktor Cheng website). Train, train, and train again