Quantitative Aptitude and Numerical Ability
Quantitative Aptitude and Numerical Ability tests speed and accuracy across arithmetic, mensuration and data interpretation. The syllabus is broad but predictable: percentage, ratio, average, profit and loss, simple and compound interest, time and work, pipes, speed, trains, boats, mixtures, mensuration and DI.
High-yield rules
- Percentage change: use fractions such as 12.5% = 1/8, 16.67% = 1/6 and 33.33% = 1/3.
- Profit and loss: discount is always calculated on marked price; profit is calculated on cost price unless stated otherwise.
- Successive change: use a + b + ab/100 for two successive percentage changes.
- Time and work: take total work as the LCM of individual times.
- Speed: distance = speed × time; convert km/h to m/s by multiplying by 5/18.
Worked example
A shopkeeper marks an article 40% above cost and gives a 25% discount. Take cost as 100. Marked price is 140. Selling price is 140 × 0.75 = 105. Profit is 105 − 100 = 5, so profit is 5% of cost. A common wrong answer is 15%, obtained by subtracting 25 from 40; that ignores the base on which discount is calculated.
Traps
- Applying discount to cost price instead of marked price.
- Treating compound interest as simple interest for more than one year.
- Forgetting to add the train’s own length when it crosses a platform.
- Mixing units in speed and mensuration questions.
- Averaging averages without weighting them by group size.
Estimate before calculating and check whether the answer is plausible.