Case math is not hard math. It's arithmetic under pressure, out loud, with someone watching. And here's the thing that reframes all of it: knowing WHAT to calculate and what the result MEANS is worth more than the arithmetic itself. Interviewers forgive a slightly rounded number delivered with a clear setup and a sharp interpretation. They don't forgive a perfectly computed answer to the wrong question.
The 8 main calculations
1. Percentage change
What it is: how much a number grew or shrank, relative to where it started.
The formula: % change = (new − old) ÷ old. Revenue went from $80M to $92M: 12 ÷ 80 = 15%.
When to use it: any "grew by / fell by" statement, growth rates off an exhibit, and the reverse version: "revenue fell 20% to $48M, what was it before?" Old = new ÷ (1 − 20%) = 48 ÷ 0.8 = $60M.
Traps: reversing by multiplying (48 × 1.2 is wrong; divide by 0.8). And percentage POINTS vs. percent: a margin going from 10% to 12% is up 2 points, which is a 20% increase. Say which one you mean.
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