In the McKinsey interview you will have to answer three different questions types - broadly speaking:
- Exhibit Interpretation
Now for Structure and Exhibit Interpretation, there is no right or wrong answer. Some answers are better than others because they are
- follow a strong communication (MECE, top-down, signposted)
That being said, there is no 100% that you can reach or the one-and-only solution/ answer. It is important that your answers display the characteristics specified above and supported well with arguments.
As for Math questions, usually, there are answers which are correct (not always 100% the same since some candidates simplify or round differently - which is ok), and others that are wrong, either due to the
- calculation approach
- calculation itself
Now, for the interviewer, the overall picture counts. Mistakes in one area need to be balanced by a strong performance in other areas. McKinsey wants to see spikes in performance in certain areas and a good enough performance in other areas.
The most common example I see almost every day: You can be strong in structure and exhibit, yet make a small mistake in the math section - overall as you might consider 80% - and still pass on to the next round.
Your phone case:
These short screening phone interviews are usually conducted by an HR person (compared to consultants, which usually do the cases). The types of questions, evaluation, etc. will all be the same. Yet the interviewer will likely record your answers (in written form) and forward them to a consultant together with their observation points. The consultant would then make the decision to move you forward or not.
Hence, your preparation does not differ from the 'real-deal' case interview.
Let me know if you need a quick evaluation of your skills and recalibration of your approach to make sure you match the interviewer's expectations. :-)