Quantitative Aptitude and Numerical Ability
This domain tests speed and accuracy with arithmetic and data interpretation: percentages, ratio and proportion, averages, profit and loss, simple and compound interest, time-work, speed-distance and tabular or graphical data. Most questions are short; the difficulty lies in choosing the right base and avoiding unnecessary calculation.
Core rules
- Percentage change is always calculated on the original base, not on the new value.
- Successive discounts are multiplicative: two 10% discounts give 0.9 × 0.9 = 0.81, i.e. a 19% discount, not 20%.
- Average speed for equal distances is the harmonic mean: 2xy/(x+y), not (x+y)/2.
- Profit% = (SP − CP)/CP × 100; mark-up is on cost, discount is on marked price.
- Simple interest = PRT/100; compound interest compounds each period.
Worked example
A shopkeeper marks an article 25% above cost and gives a 10% discount. Let cost = ₹100. Marked price = ₹125. Selling price = 90% of 125 = ₹112.50. Profit = ₹12.50, so profit% = 12.50/100 × 100 = 12.5%. A common wrong method is to subtract 10% from 25% and answer 15%.
Traps that cost marks
- Adding successive percentage changes instead of multiplying factors.
- Using the selling price as the base for profit percentage.
- Applying the arithmetic mean to average speed.
- Forgetting unit conversions in speed-distance and time-work problems.
- Rounding too early in data interpretation.