Quant Mental Math Percentages Guide
Quant mental math percentages guide for probability conversions, price changes, basis-point intuition, and trading interview drills.
Candidates who need faster percent, basis point, and probability-price conversions.
Percentages connect probability and price
Trading and market-making prompts often express probabilities, returns, and quote changes as percentages. You need quick conversions without losing the meaning of the number.
Break percentages into parts
Compute 17 percent as 10 percent plus 5 percent plus 2 percent. Compute 2.5 percent as half of 5 percent. These decompositions are reliable under time pressure.
Concrete example
For 12 percent of 350, use 10 percent plus 2 percent: 35 + 7 = 42. A rough check says 12 percent is a little above one tenth, so 42 fits.
Watch percentage change
A 20 percent fall followed by a 20 percent rise does not return to the start. Percentage changes are relative to the current base, not the old base.
Common mistakes
Candidates often mix percentage points and percent changes. Say which one you mean, especially when probabilities move from 40 percent to 45 percent.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.