Quantitative Aptitude / Numerical Ability: Bases, Rates, and Labels
Quantitative Aptitude / Numerical Ability tests arithmetic fluency and interpretation: percentages, ratio and proportion, averages, time-work, speed-distance, simple and compound interest, mixtures, number series, and data interpretation. Accuracy depends on choosing the base and the unit before calculating.
Core rules
- Percentage change = (new − old) / old × 100; percentage points compare two percentages directly.
- In time-work, add rates: combined rate = 1/A + 1/B, then time = reciprocal.
- For successive percentage changes, multiply factors: +20% then −10% gives 1.20 × 0.90 = 1.08, an 8% rise.
- In data interpretation, read the axis, legend, and year row first; calculate only the required value, using approximation when options are far apart.
- For number series, check differences, ratios, squares/cubes, and alternating patterns before assuming one rule.
Worked example
A can finish a task in 12 days and B in 18 days. Working together, how long? A’s one-day work = 1/12; B’s = 1/18. Combined = 3/36 + 2/36 = 5/36. Time = 36/5 = 7.2 days, i.e. 7 days 4.8 hours. The trap is averaging 12 and 18 to get 15; rates, not times, combine.
Traps that cost marks
Using the old value instead of the new as the percentage base; adding percentage changes instead of multiplying; mixing simple and compound interest; ignoring units such as km/h versus m/s; and over-calculating in data interpretation when options permit approximation.