Quantitative Aptitude: Speed Methods and Data Interpretation
What is tested
Numerical Ability tests simplification and approximation, number series, percentage, ratio and proportion, average, profit and loss, simple and compound interest, time–work, time–speed–distance, mensuration, elementary probability, and one or two data interpretation sets (tabular, bar, pie or caselet). The paper rewards method selection: most questions are solvable in under a minute with the right shortcut.
Shortcuts that pay
- Successive change of a% and b%: a + b + ab/100. Two successive discounts of 20% and 10% equal 20 + 10 − 200/100 = 28%.
- If a price rises by 25%, consumption must fall by 20% to keep spending constant (25 ÷ 125 × 100).
- Time and work: take the LCM of the individual days as total units of work, then convert each person to units per day.
- CI − SI for 2 years = P(r/100)².
- Adding a constant k to every observation raises the average by k but leaves the standard deviation unchanged.
Worked example
A finishes a job in 12 days and B in 18 days. They work together for 4 days, then A leaves. How long does B take to finish? LCM(12, 18) = 36 units. A does 3 units/day, B does 2 units/day; together 5 units/day. In 4 days they complete 20 units, leaving 16. B alone takes 16 ÷ 2 = 8 days, so the job finishes 12 days after the start.
Traps that cost marks
Do not compute exactly when the question says "approximately" - round early. In DI, confirm which base each percentage uses (total, previous year, or the column header). Convert ratios to fractions before comparing. Watch unit mismatches: hours against minutes, metres against kilometres, and rupees against lakh. In number series, check two-step patterns (×2 + 3, squares ± 1) before assuming a simple difference.