Case Interview Advice: Doing 'Public Math'

by Phil Stott | August 06, 2012

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This is the third in a series of posts for Vault by Nick Waugh and Kenton Kivestu on preparing for the upcoming consulting recruitment cycle. Check in for a new interview-related post each Monday until August 27th, when we will be launching the 2013 edition of the Vault Consulting 50, which will feature insider information—including tips and sample interview question—from thousands of practicing consultants at the most prestigious consulting firms in the world.

"8th grade Algebra has never been so difficult"

It is astonishing how quickly candidates are willing to admit they struggle with performing simple math in front of an interviewer. Is it stressful? Of course. Is it math they would be able to do under normal circumstances? Indeed. I’m convinced the majority of candidates struggle as a) they are not actively listening, b) they have practiced inappropriately, and c) they are so focused on doing a “math problem” and not appreciating they are solving a “business problem”.

The majority of mistakes we see on public math are students who are too busy scribbling something they fail to hear that the interviewer is asking for “percentage savings” rather than an absolute number. Additionally, they may be trying to anticipate what the interviewer is asking, already thinking of the solution or how they will calculate it, that they miss some nuance. Slowing down, clarifying the data and specific calculation requested, and otherwise active listening are keys to success in public math.

We worked with a candidate who was very nervous about public math. She had studied English as her undergraduate degree and told us how she struggled through the math section of her GMAT. She knew the public math was going to be the hardest part for her and thus worked diligently to perfect it. When we probed on how she was preparing for this anticipated hurdle, she replied she "spent 2 hours a day in the library working through old Algebra textbooks." She failed to appreciate that the challenge was not the math itself, but the environment in which she was asked to calculate it.

So many interviewees work diligently during the 10 minutes of this section of a case to get the right answer that they fail to realize what that answer means in the context of the larger business problem they are asked to solve. They get to the end of the calculation, circle the answer, look up at the interviewer for confirmation, and then take a sigh of relief. That interviewer has watched 6 other people that day struggle through this math and the entire time they are watching it performed, they are thinking "is this person going to tell me the 'so what?' when they get to the end."

Getting the right answer is important (no, actually pretty mandatory) but "distinctive" is being able to immediately apply that answer and, more importantly, it’s implications to the larger business problem at hand. Practice with old textbooks if you need to refresh your long division or percentage calculations, but remember that is only part of the public math. The most important part is the "so what?" Ensure your practice is relevant for the conditions of the interview and that you are thinking "implications" not just "right answer."

Next Week: How to End a Case Interview on a High Note


Case Interviews: The Dangers of Too Much Practice

How to Tell the Story of Your Summer Internship


Nick Waugh is currently with a private equity firm helping to form and implement strategies within its portfolio. Prior to that, he was an Engagement Manager at McKinsey and Company where he was heavily involved with recruiting out of the country's top business schools. Having been through hours of case interviews, Nick wants to see candidates more informed and thus more successful. He can be reached at

Kenton Kivestu is a Senior Product Manager with Zynga. While in business school he worked at the Boston Consulting Group and prior to that was at Google. He has helped to recruit and hire top candidates throughout his career. Kenton has channeled his desire to help others interviewing at consulting firms through his case practice tool Email Kenton at

Filed Under: Consulting | Interviewing | Job Search

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