Quantitative Aptitude
Quantitative Aptitude
This domain tests numerical fluency and interpretation: simplification, approximation, number series, percentages, ratio, averages, time-work, SI/CI, profit-loss, probability, mensuration and data interpretation.
Core rules
- For successive percentage changes, do not add or subtract the rates. Use net factor = (1 ± a)(1 ± b) − 1.
- SI = P × R × T / 100; CI = P[(1 + R/100)^T − 1]. For half-yearly compounding, halve the rate and double the time.
- In time-work, convert days to work per day: if A finishes in 12 days, A’s rate is 1/12.
- In DI, check the unit and base before calculating: lakh vs crore, percentage vs percentage points.
- Approximation questions usually reward rounding to the nearest convenient number, not exact long division.
Worked example
Two successive discounts of 20% and 10% are offered on a marked price of ₹2,000. Net factor = 0.80 × 0.90 = 0.72, so the selling price is 0.72 × 2,000 = ₹1,440. The effective discount is 28%, not 30%. The common error is to add 20% + 10%.
Traps
- Adding successive discounts or successive percentage increases.
- Using the old price as the base after a change.
- Missing that CI exceeds SI after the first year.
- Reading a DI graph’s scale incorrectly.