in terms of how to approach math in the case, this is what I would recommend:
- Repeat the question – candidates sometimes do mistakes answering the wrong question in the math part
- Present how you would like to proceed from a theoretical point of view (you may ask for time before presenting if you initially don't know how to approach the problem)
- Ask for time and perform the first computations
- Present the interviewer interim steps to keep him/her aligned – don’t just say the final number
- Continue with the computations until you find the final answer
- Propose next steps on the basis of the results you found
In terms of general math tips, this is what I would recommend:
- Use correctly 10^ powers in your math computation. For example 3.2B/723M can be transformed in 3200*10^6/732*10^6, which makes it easier to deal with math
- Ask if it is fine to approximate. When you have to deal with math in market sizing, and sometimes even in business cases, you are allowed to approximate math to simplify the computation. In the previous example, for instance, you could transform the computation in 320*10^7/73*10^7, making the overall computation faster.
- Keep good notes. One of the reasons people do mistakes with big numbers is that they don't keep their notes in order, thus forget/misreport numbers
- Divide complex math in smaller logical steps. This is something you can use for big numbers after the application of the 10^ power mentioned above. If you have to compute (96*39)*10^6, you can divide the first element in 96*40 - 96*1 = 100*40 - 4*40 - 96*1 = 4000 – 160 – 100 + 4 = 3744*10^6
- Use shortcuts for fractions. You can learn by heart fractions and thus speed up/simplify the computation - the most useful to know are 1/6, 1/7, 1/8, 1/9.
Finally, in terms of practice before the interview I would recommend to practice math under pressure - not just math. Many candidates are totally fine with calculating 67% of 67 in a quiet environment, but freeze if you ask this suddenly in a case interview.
To practice for this, try always to use a timer with a strict time constraint when you practice math – this will create pressure and help to replicate the actual environment of the interview.
Hope this helps,