Practice · Percentage Math
Case interview mental math
Percentage math is the bread and butter of case interviews. Margins, growth rates, discount impacts, cost changes — almost every case involves percentages. Speed here is the differentiator between good and great candidates.
Why interviewers test this
Percentage fluency signals comfort with business metrics. An interviewer asking 'margins dropped from 20% to 15% on €400M revenue — how much profit was lost?' isn't testing whether you can multiply. They're testing whether you can do it in 5 seconds while maintaining eye contact and continuing the conversation.
Worked examples
Revenue is €400M with 20% margins. Margins drop to 15%. How much profit was lost?
Old profit: €400M × 20% = €80M. New profit: €400M × 15% = €60M. Lost: €80M - €60M = €20M. Faster: the margin dropped 5 percentage points, so €400M × 5% = €20M.
A product costs €120. After a 15% discount, what's the sale price?
15% of €120 = €18. Sale price: €120 - €18 = €102. Shortcut: 10% is €12, 5% is €6, total discount €18.
Strategies
Use 10% as your anchor. 10% of anything is trivial — just move the decimal. Build from there.
For margin changes, calculate the delta directly: 'margin dropped 5 points on €400M = €20M lost.'
Know your fraction-to-percent conversions cold: 1/3 ≈ 33%, 1/8 = 12.5%, 1/6 ≈ 16.7%.
When multiplying by percentages, simplify first: 35% of €800M = 0.35 × 800 = 280. Or: 800 × 35 / 100 = 280.
Difficulty 1-4. Easy: single percentage of a round number. Hard: chained percentages with non-round numbers.
Practice percentage math under pressure
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