The habit that gets scored
Interviewers are not testing arithmetic contests. They are testing whether you can drive a decision with a number, and whether you notice when the number is impossible. Narrate three things on every calculation:
- The equation, in words, before the digits.
- The rounded inputs, with units.
- What the result means for the recommendation.
Silence while you multiply is fine for a few seconds. Silence followed by a number with no equation is a red flag.
The calculations that recur
Percentage change. (New − old) / old. Say which figure is the base. A churn rate that goes from 4% to 8% has doubled; it has not "gone up by 8%."
Profit. Revenue − cost, or price × volume − (fixed + variable). Do not mix a margin percentage with an absolute cost in the same subtract.
Break-even. Fixed cost / contribution per unit. State the unit. Break-even in stores is a different sentence from break-even in months.
Weighted average. Segment share × segment metric, then sum. A blended churn rate that ignores a tiny high-value segment will send you to the wrong fix.
Market sizing uses the same arithmetic on a tree. See the market sizing guide for the structure around the multiply.
Checks before you recommend
- Units still match. SAR and EUR do not add.
- The magnitude survives a plain-language test. "Each store does more revenue than the whole chain" means a zero moved.
- You can point to the one input that, if wrong, flips the decision.
Practice
Do the math inside a full case, not on a worksheet alone. The case library scores quantitative reasoning as its own line, and the interviewer will ask you to redo a step you skipped. Start with any case whose brief already contains a few numbers, and say the equation before you touch them.