Do the math out loud, calmly, fast
Consulting math isn't about being a calculator — it's about clean shortcuts, deliberate rounding, and staying composed while the interviewer watches the clock. Here's how to get quick and stay calm.
Speed comes from shortcuts, not strain
The fastest candidates aren't computing harder — they're converting to cleaner numbers and tracking their own error as they go.
Percentages & fractions
Convert on sight: 18% is just under a fifth, 12.5% is one-eighth. Reaching for the cleaner fraction turns a scary multiplication into a number you can do out loud.
Growth & CAGR shortcuts
Use the rule of 72 to read doubling time, and approximate multi-year growth without a calculator. Interviewers love a candidate who estimates compounding in their head.
Round with error tracking
Rounding is fine — losing track of which way you rounded isn't. Strong candidates round deliberately and know whether their estimate runs a little high or a little low.
“What's 18% of €4.2 billion?”
The interviewer fires it mid-case and waits. The trick is not to multiply 18 × 4.2 head-on — it's to reach for the clean fraction and round on purpose.
How a strong candidate thinks
“18% is just under a fifth. A fifth of €4.2B is €840M, so 18% is a touch below that — call it about €756M. I rounded the rate down, so the true figure is right around there, maybe a hair higher.”
The shortcuts, worked
- Percentages: 18% × €4.2B → 0.18 × 4.2 = 0.756 → ≈ €756M.
- Fractions: 12.5% of €960M = 1/8 × 960 → €120M.
- Rule of 72: growing 9% a year → 72 ÷ 9 → doubles in ~8 years.
- Big numbers: 35M × €40 → 35 × 4 = 140, add the zeros → €1.4B.
The habit
Convert to a clean fraction, do the easy multiply, add the zeros back, and say which way you rounded.
Timed sprints build the calm
Rehearsa runs the math the way the room does — out loud, against a clock — so the pressure is rehearsed long before it's real.
- Memorize the common fraction equivalents — 1/8, 1/6, 1/3, 1/7 — so percentages convert to one-step multiplications instantly.
- Drill 'big number' arithmetic by tracking zeros separately: handle the digits, then count the millions and billions back in.
- Use the rule of 72 to sanity-check growth rates: 72 ÷ growth % ≈ years to double.
- Round to clean figures on purpose and note the direction, so you can caveat 'that's a slight overestimate' with confidence.
- Practice keeping your voice steady while you compute — narrating the steps so a small slip never reads as panic.
Two minutes, ten problems
Rehearsa's math sprints serve percentages, fractions, growth rates, and big-number multiplication against a live timer — then show your speed, your accuracy, and the shortcut you should have reached for. Reps add up fast, and so does your calm.
2m
timed sprints
500+
math drills
4
shortcut families
±5%
target error band
Stop practicing. Start rehearsing.
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