I made approached the math question the wrong during a case interview, but I corrected myself. Also, I made a minor mistake - the question was about monthly profit and the data was gathered from two months, so I divided by 2 twice by mistake, but corrected myself after the interviewer's reminder. Is that a big no-no? Am I automatically rejected? Thanks for your help!
Math Error in Consulting Interview
To address the OP's original question - it depends. Our users seem to agree that making one math mistake in the case study will not ruin your chances unless you are interviewing with a major hardo. It is important to remember that they are not just looking for your quantitative skills. They are also evaluating your fit, interest, and intelligence.
I made two embarrassingly large errors in both my first and final rounds, and still received an offer and extremely positive post-interview feedback for my quantitative skills. For one case, I actually told the interviewer out loud that there were 52 months in a year, and was already half-way done with the math when the interviewer coyly asked me if I knew the difference between a week and a month. I laughed it off.
In a final round interview, the interviewer commented early-on that he was willing to chalk up my estimate to "liberal rounding". I only realized a couple minutes later that he was actually trying to point out a basic math error. Upon realize this, I said excuse me, scratched out my initial number, and promptly did a quick math shortcut to adjust all the following calculations.
While this sentiment was shared by many other users... user @FPM31 explained that a math mistake can be a deal breaker for some:
I would disagree with this; I think a lot of people (Victor Cheng comes to mind) have suggested that if you make a math error and do not catch it during a final round MBB interview, you will not get an offer (this is distinct from asking for a hint, making an error and then realizing it before the interviewer comments, etc.)
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