Quantitative Aptitude for IBPS RRB Clerk: Speed with Clean Arithmetic
What Numerical Ability Tests
The Quantitative Aptitude/Numerical Ability section (40 questions in Prelims) rewards fast, clean arithmetic: simplification and approximation, number series, quadratic equations, data interpretation and word problems on percentage, ratio, time-work, speed-distance and interest.
Core Rules
- Simplification: Apply BODMAS; convert percentages to fractions (12.5% = 1/8) and use squares up to 30 and cubes up to 15.
- Number series: Test first differences, then ratios, then square/cube patterns, then alternating or double-difference rules. Do not force one rule if the first two gaps disagree.
- Quadratic equations: Factorise ax² + bx + c = 0. For comparison, find both roots; if one equation has equal roots, compare carefully rather than assuming a relation.
- Data interpretation: Read the title, units and year labels before calculating. Use approximations only after identifying the exact quantity asked.
Worked Example
A shopkeeper marks an article 40% above cost and allows a 25% discount. Find profit percent. Let cost = 100. Marked price = 140. Selling price = 75% of 140 = 105. Profit = 5 on cost 100, so profit = 5%.
Traps
Rounding before the final step; confusing “marked price” with “selling price”; misreading “more than” versus “less than”; and in quadratic comparison, forgetting that a negative constant term gives roots of opposite signs. In DI, a common error is using total values when the question asks for a ratio or average.