Quantitative Aptitude and Numerical Ability
What this domain tests
Quantitative aptitude checks numerical ability through arithmetic and applied reasoning: percentage, ratio and proportion, average, profit and loss, simple and compound interest, time and work, speed–distance–time, mensuration and data interpretation. OSSSC clerk-level papers usually avoid high-level mathematics; the challenge is accurate calculation under time pressure without a calculator.
Rules that pay off
- Percentage change uses the original base: change/original × 100.
- Successive discounts are multiplied, not added: a 20% and 10% discount equals 0.8 × 0.9 = 0.72, i.e. 28% off.
- For profit/loss, take CP = 100 when a percentage is given and no amount is fixed.
- In time and work, convert days to work per day: if A finishes in 12 days, A does 1/12 per day.
- For two trains or boats, add speeds for opposite directions and subtract for same direction.
- In CI, use A = P(1 + r/100)^n; SI uses P × r × t/100.
Worked example
A shopkeeper marks an article 25% above cost and allows a 10% discount. Find the profit percentage. Let CP = ₹100. Marked price = ₹125. Selling price after 10% discount = 125 × 0.90 = ₹112.50. Profit = 112.50 − 100 = ₹12.50. Profit% = 12.50/100 × 100 = 12.5%.
Traps
Do not calculate discount on cost price; discount is on marked price. Do not add successive percentage changes. In average questions, remember that adding a value changes both total and count. In time and work, do not average completion times; average rates instead.