Training · CO-2 · Lesson 05 of 06

Mental maths

The rule

Case maths is fast arithmetic with round numbers. Speed comes from practising the same operations until they are automatic. Drill three-digit multiplication until it is reflex, so your attention stays on the logic.

Why it matters

Case interviews require quick, accurate calculations without a calculator. Errors undermine credibility; slow maths eats time.

Core techniques

Master shortcuts that come up repeatedly.

·Round numbers and adjust afterwards.
·Use scientific notation for large numbers.
·Break multiplications into parts.
·Know percentages as fractions.

Worked examples

Practise common calculations until automatic.

Shortcuts

15% of 240: 10% (24) + 5% (12) = 36
48 × 25: 48 × 100 / 4 = 1,200
3.2m × 450: 3.2 × 4.5 × 10⁸ = 1.44bn
1/8: 12.5%
Growth 10% for 7 years: ≈ 2x (rule of 72)

Communicating calculations

Talk through steps, write neatly and label units. Check the order of magnitude before stating answers.

Practice routine

Drill ten minutes daily with timed questions. Use case-maths apps and practise percentages, growth rates and breakeven calculations.

Hold onto this

·Work in thousands and millions; never carry more than three significant digits.
·Know percentage change, margins and break-even volumes cold.
·Narrate each step so an arithmetic slip does not read as a logic error.

Progress saved on this device

Education, not financial advice. Figures may be simplified for illustration.