Any tips to do math calculations fast ?

In order to perform math calculations faster, here are some tips you can use:

• Increase the number of steps in your calculations and turning complex operations into simple ones: It seems counter-intuitive but let me explain. Let's consider the following example: 67 x 89. If we were to calculate this the traditional way, then we would waste considerable time which will have a negative impact on our overall performance during the interview. Instead, by increasing the number of steps in the calculations, we can bring the operations to calculations that are easier to perform. Still using the example above:  67 x 89 = 67 x (90 - 1) = 67 x 90 - 67 = (60 + 7) x 90 - 67 = 60 x 90 + 7 x 90 - 60 - 7. The operation seems long but all the elements are simple calculations that you can perform quickly. Thus 67 x 89 = 5400 + 630 - 60 - 7 = 5963. Tying it back to the initial proposal, we turned multiplication into addition and substraction, which is much easier to do.
• Know shortcuts and certain computations to speed up the calculation: To be exhaustive, below is a list of things that should be memorized beforehand:
  • Multiplication tables: They should be memorized and even extended to include multiplication tables up to 15 or 16 (or even 20 if you're brave enough);
  • Basic squares and cubes: Ideally all of them from 1 to 10 should be known to help you speed up some calculations;
  • Basic fractions: Mainly from 1 to 1/10. As weird as this sounds, fractions come in extremely handy when dealing with percentages: 25% can be replaced with 1/4 and 66% can be replaced with 2/3, instead of doing the calculations by hand;
  • Shortcuts: Multiplying numbers by 5 and ending with certain digits, multiplying numbers by 11, certain shortcuts relative to addition and substraction should be known. A quick Google search on math tricks should uncover most of these for you.
• Use mental math apps and online tools to practice mental math: Nothing beats live practice and it's best to do it in an environment or with a tool that simulates the sense of urgency in a case interview. As such online mental math tools such as the one on PrepLounge and Victor Cheng's tool (http://www.caseinterview.com/mental-math) are good tools to practice with. There are many mental math apps available on the App Store and the Google Store but they are not really professional nor useful (Unless we're talking about those provided by Magoosh or other websites that offer training in numerical tests).
• PRACTICE, PRACTICE, PRACTICE: need I say more? :)

Best of luck.

Let me share a secret: There's no real benefit in the context of case studies in doing maths relatively fast. In fact, people worrying they're being too slow tend to make stupid mistakes by skipping steps. 

"Being slow" at maths in cases generally means the person loses their way, or is trying to solve the wrong problem. Obviously it's a plus if you're a human Excel, but for most people the perfect case interview is simply clearly written down and talked through maths, starting with the problem in words, followed by numbers. 

I always advise people to take their time, work through complicated numbers in detail on a separate sheet of paper if necessary and for 95% of people there will be no problem with speed.

I used a very interesting book: 
https://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Calculation/dp/0307338401

Hi Khaoula,

I'm a former McKinsey consultant, and I have a number of methods and recommendtions for doing caculations more efficiently, which leads to much faster calcuations and fewer mistakes.

I disagree with some other responses which say that doing math quickly isn't a benefit in an interview. I have had numeous peoeple who said htey used my methods to solve the quant problmes in Case Interviews quickly, and that the interviewer was defintely impressed. While it's true, that if you try to rush the math, you might make a mistake, you can learn specific methods that are simpler, faster and less error prone.

For example, what is 120 Million times 250? This could come up if they tell you a company sells 120 Million units per year, at $250 per unit. A typical approach would be to use Long-hand multiplication, or to caculate 12 × 25 and count zeroes. These both tend to be somewhat slow, and error prone, especially with counting zeroes.

Here's an efficient methods: 

• Decompose 250 into ¼ × 1,000
• ¼ × 120 Million = 30 Million
• 30 Million × 1,000 = 30 Billion (Million × Thousand = Billion)

There are a number of other methods like this, and I wrote some articles in the Bootcamp section on quant skills required in Case Interviews.

I have also created the FastMath Ace the Case online course specifically to teach the quant skills needed to succeed in Case Interviews — you can access the course here: http://www.fastmath.net/ace-the-case/?pc=preplounge_bc_001

Enjoy,
Matthew

There are a couple of good apps on the Apple App Store that you can use for practice.

Search "Case Math" and you'll find a few.