the most common formulas you need to know as part of the interview process are:
- Breakeven point
- Rule of 72 (usually related to calculation of CAGR)
- Perpetuity formula and general NPV formula
Besides that, you should be familiar with simple and compound interests.
If you find challenging to structure something different from that, I would recommend to post the questions for which you find difficult to understand the formula for further support.