Numerical Ability and Mental Aptitude
Numerical ability and mental aptitude tests speed with arithmetic and quantitative reasoning: percentage, ratio and proportion, average, profit and loss, simple and compound interest, time and work, speed and distance, mixtures, mensuration, and data interpretation. The exam rewards clean method and unit discipline more than long calculation.
Core rules
- Percentage change uses a multiplier: a 20% increase is ×1.20; a 20% decrease is ×0.80.
- Successive changes multiply, so two 10% increases give ×1.10×1.10 = ×1.21, i.e. 21%.
- Average speed = total distance ÷ total time, not the arithmetic mean of speeds.
- In trains, crossing a platform adds the train length to the platform length.
- For CI, use A = P(1 + r/n)^(nt); for SI, I = PRT/100.
Worked example
A 120 m train crosses a 180 m platform in 15 s. Total distance = 120 + 180 = 300 m. Speed = 300 ÷ 15 = 20 m/s. Convert to km/h: 20 × 18/5 = 72 km/h. The platform length is included because the whole train must clear it; using 120 m alone would give 28.8 km/h and is wrong.
Traps
Do not add successive percentages; multiply multipliers. Do not average speeds when times differ. Convert minutes to hours before using km/h, and remember 1 m/s = 3.6 km/h. In profit and loss, take profit percentage on cost price unless the question says otherwise. In time and work, add rates, not times. In CI, count compounding periods carefully-half-yearly compounding doubles n and halves the rate.