the problem you mentioned is very common. The majority of the people are actually ok with the math in a case in normal conditions. However they “suddenly” make a lot of mistakes once in the case: missing zeros, simple math errors in divisions, wrong percentage, etc.
Why is that the case?
There are two main reasons I identified:
1) Practiced math without a tough time constraints.
You may be a math PhD, but unless you train your math with a tough time constraint during the prep for consulting interview, you will likely end doing mistakes. Why? Because in a consulting interview you will face something you haven’t faced before: pressure. You know you are seen and judged for each mistake you will do, and that you have to come up with a solution quickly. This makes easier to do mistakes. Unless you have trained for that in advance.
What is a then good solution for that? Start to practice math exercises with a time constraint that is unfeasible for you to satisfy initially. Eg: if the average time you spend for percentage exercises is – say - 15 seconds, give yourself 12 second for the following exercise. Not only you will learn how to speed up your math, but forcing yourself to solve something in an “impossible” timing will create pressure and will help you to get accustomed to it.
2) Lack of structure in the math.
Many people start with math computation jumping to the numbers. That’s not the best approach to follow.
You should treat math in the same way you deal with the beginning of the case. You don’t start a case going in details in one area, but presenting the overall approach you have, and THEN moving to the details. In the math part you should do the same thing: present first your theoretical approach (the forumla you are going to use), and THEN move to the numbers.
Why is that important for math mistakes, you may ask? At the end, you will still have to go through the math anyway.
That’s correct, but there is a caveat. Your math mistakes, as mentioned before, are not due to lack of knowledge, but to uncontrolled pressure. Presenting first your approach, and aligning with the interviewer, will not only allow you to know if your thoughts are correct, but also help to release pressure. Indeed, once presented your approach, you will understand before the actual math if your logic is correct or not. In this way, your following math will also improve due to the fact you know you are doing the right thing.
Hope this helps,