Numerical parts of the case study are not designed to be standalone. The objective is to get your level of comfort with the analytical elements of the case, test your problem solving abilities and to know that you can do some basic calculations (with one or two tricky elements to solve). There are a few ways to prep for this
1. Get quick at basic arithmetic - practice multiplications, divisions and squares etc. often. It will take a few hours of practice for your brain to get used to solving these much more quickly which frees up time to focus on problem structuring
2. Spend time upfront thinking about and structuring the problem. The math part of most cases is not the toughest part, the toughest part is truly understanding what the question is asking for and then structuring the solution accordingly
3. Practice different types of problems - some are straightforward estimations (e.g., calculate the number of customers required to break even), some require more time (e.g., how many cars pass through a given bridge at one time) and some are a hybrid. You can never predict what you will get but more practice will let you get comfortable that you can solve these problems.
If you give yourself the flexibility to approach each problem on its own merits vs following a structured framework you will be a lot better off. To do so its the usual answer of practice till you get comfortable with these problem types