Quantitative Aptitude / Numerical Ability
What is tested
Numerical Ability covers simplification, approximation, number series, quadratic equations, arithmetic and data interpretation. Arithmetic topics include percentage, ratio-proportion, average, time-work, speed-distance, simple and compound interest, profit-loss, partnership, mixtures and mensuration.
Core rules
- Percentage change uses the original base: a rise from 80 to 100 is 25%, not 20%.
- For successive percentage changes, use multipliers: +20% then -10% gives 1.2 × 0.9 = 1.08, i.e. +8%.
- Time and work: if A finishes in a days and B in b days, together they take ab/(a+b) days.
- Compound interest: A = P(1+r/n)^(nt); simple interest is PRT/100.
Worked example
A alone can complete a contract in 12 days and B alone in 18 days. They work together for 4 days, then A leaves. How long does B take to finish? Together rate = 1/12 + 1/18 = 5/36 per day. In 4 days they finish 20/36 = 5/9. Remaining = 4/9. B's rate = 1/18, so time = (4/9) × 18 = 8 days. Answer: 8 days.
Traps
Using the new value as the base in percentage questions; mixing SI and CI; forgetting unit conversion in speed or mensuration; and marking an approximation answer without checking whether the question asks for exact value.