Consulting Math Plain Number Calculation

Plain Number Calculation is the most basic building-block in improving your consulting math. At the end of the day, you need to be able to perform the striped-down number calculations, and a lot of them.

There are 8 types of plain number calculation questions that you will mostly encounter. They can range anywhere from simple conversions to big number calculation. Honing your mental arithmetic is the key to solving them!

  A quick glance

  • Type 1: Percentage
  • Type 2: Multiplications
  • Type 3: Increase in Percentage
  • Type 4: Decrease in Percentage
  • Type 5: Currency Conversion-From USD
  • Type 6: Currency Conversion-To USD
  • Type 7: Time Conversion
  • Type 8: Big Number Comfort

As I mentioned in the Mental Math article, it’s usually acceptable to have up to 5% margin for errors, even with Plain Number Calculation.  All answer keys in this practice drills are absolutely correct with 0% margin of error. In the MCP spirit, we also break down Plain-Number calculations into 8 types. By isolating each type of calculations, that brain muscle group can be practice over and over continuously to reach the growth point.

Let’s go into each one of them.

Remember that accuracy is as important (if not more) as speed. Try to be correct (meaning staying within 5% of the answer key) as many time as possible and then gradually improve your speed.

Type 1: Percentages

In a consulting context, people like to look at values, not only in absolute, but also relative to
each other. Percentages become a very useful tool. 

As a consultant, you will very often find yourself doing back-of-the-envelope percentage calculations either in problem-solving meetings, in client interviews, or in your own analyses. Therefore, it’s not surprising that this type of calculation is very common in Case Interviews and entrance tests (McKinsey PST & BCG Potential Test).

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Type 1: Percentages​

1 / 5

7.5% of 85?

2 / 5

20% of 1,250?

3 / 5

35% of 40,000?

4 / 5

4% of 1,340?

5 / 5

33% of 2.5 billion?

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Type 2: Multiplication

Besides additions and subtractions, multiplications are another important building block of Plain Number Calculation. Multiplications are not only common in real life, but also in every aspect of business and consulting.

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Type 2: Multiplication

1 / 5

2.5 x 3,400?

2 / 5

85 x 23,000?

3 / 5

2,350 x 8?

4 / 5

4 x 920?

5 / 5

350 x 12?

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Type 3: Increase in Percentage

This is a twist version of type 1 above, but it is slightly different. Let’s try some more practice with Plain Number Calculation!

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Type 3: Increase in Percentage

1 / 5

900 increased by 54% would be…?

2 / 5

85,200 increased by 6% would be…?

3 / 5

6,200 increased by 15% would be…?

4 / 5

2,400,000 increased by 75 % would be…?

5 / 5

265 increased by 0.2 % would be…?

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Type 4: Decrease in Percentage

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Type 4: Decrease in Percentage

1 / 5

7,400 decreased by 22% would be…?

2 / 5

575 decreased by 0.6 % would be…?

3 / 5

200 decreased by 45% would be…?

4 / 5

8,500,000 decreased by 35 % would be…?

5 / 5

73,500 decreased by 8% would be…?

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Type 5: Currency Conversion - From USD

The world is getting smaller and international business is increasingly common. Along with that comes the need to do cross-country currency conversions in real consulting work. This is very common in Plain Number Calculation.

Below are the acceptable approximate conversion rates between US Dollars and the 7 most popular currencies in the world as of 2019. These conversion rates fluctuate day to day; however, there are still generally-accepted rates in most cases. These estimates are acceptable
in Case Interviews unless the interviewer specifically instructs you to use more accurate rates.

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Type 5: Currency Conversion - From USD

1 / 7

$300 USD equals … EUR?

2 / 7

$15,000 USD equals … GBP?

3 / 7

$25,200 USD equals … INR?

4 / 7

$12,500 USD equals … JPY?

5 / 7

$7,500 USD equals … AUD?

6 / 7

$650 USD equals … CNY?

7 / 7

$2 million USD equals … SGD?

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Type 6: Currency Conversion - To USD

A twist version of the above. The trick is to use division instead of multiplication. Let’s try a few
practice ones.

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Type 6: Currency Conversion - To USD​

1 / 7

7,020 CNY equal to…USD?

2 / 7

7,400 AUD equal to…USD?

3 / 7

250 GBP equals to…USD?

4 / 7

600,000 SGD equal to…USD?

5 / 7

500 EUR equal to…USD?

6 / 7

15,400 INR equal to…USD?

7 / 7

1,250,000 JPY equal to…USD?

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Type 7: Time Conversion

Though not very popular in Case Interviews and entrance tests (McKinsey PST & BCG Potential Test), calculations involving time units can be very tricky and time-consuming. Most units of time are easy to imagine and sense but very hard to do calculations with; this is because of their odd nature. There are only 60, not 100, seconds in a minute, 60, not 100, minutes in an hour, 24, not 25, hours in a day, 30, not 50, days in a month, 52, not 50, weeks in a year, etc. Being skilled in this type of calculations will add value to your interviewing and test taking skillset!

When doing this Plain Number Calculation, please note that there are two different ways to
calculate hours per week:

  • 24/7: There are 24 hours per day and 7 days per week.
  • Standard work hour: There are 8 hours per working day, and 5 working days per week.

Assuming that there are 30 days per month and 365 days per year.

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Type 7: Time Conversion​

1 / 5

15 items per hour equal to…items per month? (Standard work hours)

2 / 5

1,800 USD per month equal to …USD per day? (Standard work hours)

3 / 5

75,000 items per year equal to…items per week? (Standard work hours)

4 / 5

25 USD per day equal to…USD per year? (24/7)

5 / 5

120 items per day equal to…items per hour? (24/7)

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Type 8: Big Number Comfort

When people think of Management Consulting, usually they think of big projects, big clients, and big numbers. Almost all numbers floating around each project are big: big revenue, big production volume, big number of FTE (Full-time Equivalent) employees, big investments, big assets, etc. It is essential for all consultants to be comfortable working with big numbers. They have to be able to sense how big “big numbers” really are and perform big calculations with them.

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Type 8: Big Number Comfort​

1 / 5

0.06 trillion equals to…million?

2 / 5

1.5 trillion equals to…thousand?

3 / 5

500 thousand equals to…million?

4 / 5

6400 billion equals to…trillion?

5 / 5

450,000 thousand equals to… million?

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