Fast, clean, and checked
Mental-math habits, large numbers (million, billion, lakh, crore), calculating out loud, and sanity checks.
Key takeaways
- Always sanity-check. After any answer, ask: is this number possible?
- Write large numbers in short form: 3 million is 3 M, 45 billion is 45 B.
- 1,000 times 1,000 is a million. 1,000 times a million is a billion.
- In India, numbers are often given in lakh and crore. 1 lakh is 100,000.
Speed in case math comes from a few habits, not from talent. Round sensibly, then adjust: 19 percent of 412 is close to 20 percent of 400, which is 80. Both roundings were small, so the real answer is close to 80 (it is about 78). Use easy numbers and powers of ten, and write the units next to every number so you never lose a zero.
Large numbers without mistakes
Keep the zeros under control
- Write large numbers in short form: 3 million is 3 M, 45 billion is 45 B. Then multiply the short numbers and add the zeros at the end.
- 1,000 times 1,000 is a million. 1,000 times a million is a billion.
- In India, numbers are often given in lakh and crore. 1 lakh is 100,000. 1 crore is 10 million (100 lakh). So INR 250 crore is INR 2.5 billion.
- Useful fractions: one third is about 33 percent, one sixth about 17 percent, one seventh about 14 percent, one eighth 12.5 percent, one ninth about 11 percent.
A company in Mumbai has revenue of INR 3.2 crore. How much is that in millions of rupees?
Calculate so the interviewer can follow
- Say what you are about to do before you do it: "I will multiply the 2 million customers by the 30 dollars each."
- Write the numbers in a line on your paper. The interviewer is scoring how you reason, not only the final figure.
- Keep a label on every number, so a 5 is "5 million units", never a lonely 5.
- Give the answer with its unit and a quick check: "That is 60 million dollars in revenue, which seems reasonable for a mid-size brand."
Key idea
Always sanity-check. After any answer, ask: is this number possible? If you size a city's coffee market and get a figure bigger than the whole country's economy, there is an error somewhere. Catching it yourself is a strong signal.
A product's price rises from 40 to 50. What percentage increase is that? Answer as a number of percent.
A firm has revenue of 5,000,000 and costs of 3,500,000. What is its profit margin in percent?
Sources for this lesson (1)
- Recognized public explanations of case-interview concepts and frameworks
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