Quantitative Aptitude
What this domain tests
Numerical ability carries 35 questions in the prelims: simplification, number series, percentage, ratio, average, interest, time-work, speed-distance, mensuration, probability and data interpretation. Speed matters more than depth.
Core rules
- Percentage and ratio - convert statements into a base value; "A is 25% more than B" means A = 1.25B, so B is 20% less than A.
- Interest - SI = PRT/100; CI = P(1 + r/100)^n − P. The CI − SI difference for two years is P(r/100)².
- Time and work - use rates, not times: if A finishes in 12 days and B in 18, their combined rate is 1/12 + 1/18 = 5/36, so together they take 7.2 days.
- Data interpretation - read the axes and units first, and use approximation anchored to the options instead of exact long division.
- Number series - test differences, then ratios, then a mixed pattern (×2 + 3, squares ± 1) before assuming a new rule.
Worked example
The difference between compound and simple interest on a sum at 6% per annum for two years is ₹180. Using P(r/100)² = 180: P × 0.0036 = 180, so P = 180/0.0036 = ₹50,000. Verify: SI on ₹50,000 at 6% for two years = ₹6,000; CI = 50,000 × 1.06² − 50,000 = 56,180 − 50,000 = ₹6,180; difference ₹180.
Traps
Using the annual formula when interest is compounded half-yearly (halve r, double n); confusing percentage points with percentage change; ignoring that average questions often ask for the new average after one item is replaced or added.