Staying precise with numbers when an interviewer is watching

The pressure problem

Most candidates can do the math. The difference between a good and great candidate is whether they can maintain accuracy while an interviewer watches — and while thinking about what the numbers mean.

Why errors happen under pressure

Quantitative mistakes in case interviews usually come from one of three sources:

• Rushing — Trying to calculate faster than your brain can process
• Losing track — Forgetting which number you're working with
• Not narrating — Keeping calculations in your head without verbalising the approach

The structured narration approach

Top candidates never calculate in silence. Instead, they follow this rhythm:

1. Announce what you're calculating: "Now I'm going to estimate the addressable market by multiplying..."
2. Round clearly: "Let's call 37 million roughly 40 million for simplicity."
3. Calculate step by step: "40 million × 25% gives us 10 million."
4. Interpret the result: "So the addressable market is approximately 10 million customers."

Daily drills for mental arithmetic

Practise these for 5 minutes each day before your interview prep:

• Multiplication of two-digit numbers
• Percentage calculations (what is 15% of 240?)
• Market sizing estimates (estimate the total market)
• Growth rate projections (if something grows 8% annually for 5 years)