Math plays an important role in case interviews conducted by top consulting firms such as McKinsey, BCG, and Bain. These firms are looking for candidates who can quickly and accurately perform basic mathematical calculations under pressure.
The math used in a case interview is usually simple arithmetic, including basic addition, subtraction, multiplication, and division. Candidates may also be expected to perform calculations involving fractions, decimals, and percentages. In addition, some case interviews may require more advanced mathematical skills, such as calculating compound interest or determining the present value of an investment. The latter is especially relevant for specialized hires.
A common question I often receive and see on forums is if you are allowed to use a calculator in a case interview. In this article, I want to answer this and provide some tips on how to deal with case math calculations.
Why are consulting firms testing your math skills?
The use of math in a case interview is evaluated by top consulting firms because it provides insight into a candidate’s ability to think critically and solve problems. These firms are looking for candidates who can make quick, accurate calculations and use their mathematical skills to support their arguments and recommendations. The use of math also helps to assess a candidate’s ability to work under pressure and make decisions in real-time. By requiring candidates to perform mathematical calculations during a case interview, top consulting firms are able to get a better understanding of a candidate’s analytical skills, problem-solving abilities, and overall aptitude for consulting work.
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You cannot use a calculator in a case interview
Now, can you bring your calculator to the interviews or use one during a Zoom interview? The short answer is no (and you should also not try to game the system by calculating outside of the webcam’s view).
Whether you have already learned that calculators are not allowed or not while preparing for a case interview, this can be a daunting experience for those who are used to relying on technology to help them perform basic mathematical calculations, which is actually everyone. 🙂 You have likely not done any mental math since early high school and since then relied on calculators and spreadsheet software such as Excel.
If you are using a calculator, consulting firms are not able to evaluate your performance and skill in this regard. While 99% of your quantitative work on the job is done using Excel, there might be situations where you need to demonstrate strong mental math skills, for instance, during a client meeting to make some quick estimations or to defend or challenge statements.
To be fully comprehensive, I once coached a candidate for a U.S. McKinsey office who was allowed to use a calculator during the interview. Out of more than 1350 sessions I have done at the time of writing with hundreds of clients, she was the only one across all MBBs, tier-2 firms, Big 4, in-house consulting, and boutique consulting firms that was allowed a calculator and she did not know either why she had that privilege (she got the offer btw).
For anyone else, there is likely no way around it and you should spend some time getting back to speed with mental calculations.
What should you do instead?
Luckily, there are simple tips and techniques that can help you improve your mental math skills and practice them regularly.
Simple tips for mental math
- Learn the basics. One of the most important things you can do to prepare for a case interview is to learn how to sum, subtract, multiply, and divide double-digit numbers quickly and accurately. Once you have remembered, for instance, how to perform divisions again, it’s time to simplify the process.
- Round numbers to simplify calculations. For example, instead of calculating 29 x 4, round 29 to 30 and multiply it by 4. The answer is 120, which is close to the actual answer of 116. If you are multiplying or summing numbers, try rounding one number up and the other one down to cancel out the effects. If you are dividing or subtracting, round both numbers in the same direction. Try to stay within a 5-10% margin.
- Break down problems into smaller parts. Instead of trying to solve a complex problem all at once, break it down into smaller, more manageable parts. For example, if you need to multiply 25 x 8, first multiply 25 x 2 (which is 50) and then multiply 50 x 4 (which is 200). Alternatively, you could also calculate 25 x 10 – 25 x 2 = 250 – 50 = 200.
- Learn how to deal with zeros. Working with zeros can be a challenge, especially when multiplying or dividing large numbers. One strategy for avoiding mistakes when working with zeros is to always use 10^power instead of zeros. For example, 200 x 8000 = 2 * 10^2 x 8 * 10^3 = 2 x 8 * 10^(2+3) = 16 * 10^5. This may look complicated at first, but with practice, it becomes much easier and helps you avoid losing zeros or making other errors.
- Practice with simple calculations before moving on to more complex ones. Start with simple problems and gradually increase the difficulty level as you become more comfortable with mental math. For instance, our case math course follows a logical sequence from simple calculations to intermediate and complex ones.
- Learn how to take clean and logical notes and keep track of your calculations. Write down everything including the input numbers, the calculation steps, intermediate results, and the final result when doing mental calculations. Find a system of note-taking that works for you. Your calculations will be more organized and if you make a mistake with a track record, you can quickly fix it and move on.
More advanced tips for mental math
- Use estimation to sanity-check answers. When you’re finished with a mental math problem, use estimation to see if your answer is reasonable. For example, if you calculate that 123 x 28 is 3,444, check to see if the answer is reasonable. Since 28 is close to 30 and 123 is close to 120, you can estimate that the answer should be around 3,600. You could also frontload this before actually going through the accurate calculation. In the best case, interviewers are already happy with the approximate result (see Rounding above)
- Learn mental math shortcuts. For instance, break calculations apart. For example, you can quickly calculate 12 x 15 by recognizing that 12 x 10 is 120 and 12 x 5 is 60, so the answer is 180.
- Memorize multiplication tables, squares, and cubes. For instance, memorize the division table up to x/20, such that you are able to quickly translate fractions (e.g., 6/7 = 86%). This will help you quickly calculate percentage problems without the need for a calculator. Memorize key numbers like powers of 2, 5, and 10, which can be helpful in solving problems.
Techniques for practicing mental math
- Practice mental math daily. Like any skill, practicing mental math regularly is crucial for improvement. Burn through drills using timers to create a more realistic scenario. Set yourself the goal to accurately complete a certain amount of calculations in a given timeframe.
- Use real-life situations to practice mental math. Try to incorporate mental math into everyday activities, like calculating the cost of groceries, determining the tip for a restaurant bill, or calculating the time and distance for a trip.
- Practice mental math with a partner or group. Practicing mental math with others can be fun and motivating. You can compete against each other or work together to solve problems.
- Track progress and celebrate small improvements. Keep track of your progress by setting goals and tracking your improvement over time. Celebrate small improvements to stay motivated. Create an error log to collect all the errors you have made. This will help you learn, prevent similar errors in the future, and guide your preparation (e.g., if you keep making the same mistakes in one area or type of calculation, it might be worthwhile to focus more on that area).
Remember, the key is to practice, practice, and practice again until you are confident in your abilities.
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Overcoming mental math challenges
If you struggle with mental math, you’re not alone. Math anxiety and working memory limitations are common challenges that many candidates face during case interviews and preparation. To overcome these challenges, try these tips:
- Take deep breaths and stay calm. When you feel anxious or overwhelmed, take deep breaths and focus on staying calm. Remember that mental math is a skill that can be improved with practice.
- Break down problems into smaller parts. If you’re struggling with a complex problem, break it down into smaller parts. This can help you stay focused and avoid feeling overwhelmed.
- Use mnemonic devices to remember key information. Mnemonic devices, like acronyms, can be helpful for remembering key numbers or formulas.
Seek help if you need it. If you’re struggling with mental math, don’t hesitate to seek help from a tutor or coach. They can provide personalized guidance and support to help you improve your skills.
In conclusion, the use of calculators is not allowed in case interviews for a reason. Among many other things, consulting firms want to test your basic mathematical skills and assess your ability to think on your feet in a stressful situation.
Remember to practice regularly, use real-life situations to practice, and track and celebrate small improvements along the way. With dedication and perseverance, you can become a mental math whiz in around four weeks. Case interview math is never too difficult yet requires a certain set of skills that need to be demonstrated under time pressure in a challenging environment.
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