Most case interview math is percentages. The interviewer asks what share of sales a product makes up, how fast revenue grew, or what a price change does to the margin. There’s no calculator, so you need a few moves you can trust.
- Share of a total: part divided by total. Simplify before you divide: 24 / 150 is 8 / 50, which is 16%.
- Growth rate: (new minus old) divided by old. Always divide by the starting number.
- Percent vs percentage points: a margin that moves from 8% to 10% rose 2 percentage points, which is a 25% increase. Interviewers listen for which one you mean.
- Compounding: two years at 10% is 1.1 × 1.1 = 1.21, a 21% rise, not 20%.
Say your rounding out loud. "Call it 5,000" is fine in an interview if you correct for it at the end. All companies and figures in this course are fictional.