when approaching the quant part of a case, it is important to think of it as a "mini case", as the approach would be exactly the same:
- Take some time at the beginning to structure the problem. Define the question clearly (in this case: What do you want to calculate, using the data you have).
- Then develop a framework (the equation) and take the interviewer along by telling them what you will calculate. In a face-to-face meeting, you can show them the equation to take them along, through zoom you need to overinvest in this stage. End with the question on whether this makes sense or the interview would have any comments.
- Once aligned on the equation, execute the math. During the execution, pay attention to potential short-cuts. Often you don't need to make the complex written division if you see a short cut. Confirm with the interviewer if it's OK to make simplifications (e.g. calculating with +-5% accuracy, multiplying with 100 instead of 97, etc), rather than calculating the last digit. If you are insecure about your mental and written math skills and about seeing these shortcuts, there are many resources on practicing this. GMAT resources are a good start.
- Before you communicate your results, do a quick mental sense check. Is the number in the right ballpark, or does it feel completely out of whack? If it feels wrong, tell the interviewer about your result, but that it doesn't feel right so you want to double-check for potential sources of error.
- Communicate your results and give the interviewer the chance to react before using it in your next steps.
To practice this (as opposed to just the execution), practice with the available cases here on Preplounge and tell your case partners to pay special attention of your handling of the quant section.
I hope this helps. If you need someone to practice this in a systematic way, feel free to DM me.