Interview Mental Math Common Mistakes
Common mental math mistakes in quant interviews, including speed errors, sign errors, denominator errors, unit mistakes, and weak review loops.
Candidates who make repeat arithmetic, sign, unit, or speed errors.
Speed errors
Rushing creates dropped signs, missed costs, and skipped denominators. Speed should be added only after a calculation type is accurate at a slower pace.
Sign and payoff errors
Expected value and PnL prompts often fail because losses are not made negative or costs are omitted. Write net payoffs before multiplying.
Concrete example
If a bet wins 3 and loses 1, the losing branch must be -1. Averaging 3 and 1 instead of 3 and -1 changes the decision completely.
Unit errors
Daily rates, annual rates, per-user values, and totals should not be mixed in the same line. Keep units visible until the final answer.
Common review mistake
Candidates often mark an answer wrong but do not classify why. An error log should turn each repeated mistake into a specific repair drill.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.