Quantitative Aptitude / Numerical Ability
Numerical Ability rewards clean method and approximation discipline. The paper mixes simplification, number series, arithmetic and data interpretation, and DI usually carries the largest share of marks.
Core rules
- Simplification: apply BODMAS and memorise fraction equivalents - 12.5% = 1/8, 16.66% = 1/6, 6.25% = 1/16.
- Series: test differences first, then ratios, then squares and cubes, then alternating or double-difference patterns.
- Quadratic equations: solve both, then compare roots; the equation with both roots smaller is the smaller one.
- Compound versus simple interest: CI − SI for 2 years = P(r/100)²; for 3 years = P(r/100)²(3 + r/100).
- Time and work: total work = LCM of the individual days; efficiency is the inverse of time.
- Successive percentage change: a% then b% gives a + b + ab/100.
Worked example
The difference between CI and SI on a sum for 2 years at 10% per annum is ₹40. Find the sum. CI − SI (2 years) = P(r/100)² = P(0.1)² = 0.01P. 0.01P = 40, so P = ₹4,000. Check: SI = 4000 × 10 × 2/100 = ₹800; CI = 4000(1.1² − 1) = ₹840; difference ₹40.
Traps
Using the 2-year difference formula for a 3-year question; confusing percentage points with percent; averaging averages; ignoring lakh and crore units in DI tables; rounding too early and losing the exact option.