Numerical Ability: Method, Then Calculation
Numerical Ability has 40 questions on simplification and approximation, number series, quadratic equations, arithmetic word problems and data interpretation. The paper is speed-scored: one clean method per question matters more than a long derivation.
Core rules
- Simplification: apply BODMAS; approximate only when answer options are far apart.
- Percentage change: successive changes use a + b + ab/100, with declines negative.
- Ratio: convert to parts; one part = total ÷ number of parts.
- Time and work: take total work as the LCM of days; efficiency is the inverse of time.
- Interest: SI = PRT/100; CI = P[(1 + R/100)^T − 1].
- DI: note units (lakh, crore, %), and whether the question asks for share of total or growth over the previous year.
Worked example
A shirt has a marked price of ₹2,500. Two successive discounts of 20% and 10% are given. Find the single equivalent discount and the selling price. Take MP = 100. After 20% discount: 80. After 10%: 80 × 0.90 = 72. Single discount = 100 − 72 = 28%. On ₹2,500: SP = 2,500 × 0.72 = ₹1,800. Check: 20% of 2,500 = 500, leaving 2,000; 10% of 2,000 = 200; final 1,800. The single discount is 28%, not 30%, because the second discount applies to the reduced price.
Traps
- Adding 20% + 10% and calling it 30%.
- Taking discount on selling price instead of marked price.
- Using the SI formula for CI, or compounding annually when the question says half-yearly.
- In DI, confusing "percentage of total" with "percentage more than previous year".