Quantitative Aptitude
What the section tests
Quantitative Aptitude tests speed and accuracy across arithmetic and data interpretation: percentage, ratio and proportion, averages, profit and loss, simple and compound interest, time-work, speed-distance, mixtures, number series, simplification and approximation, quadratic equations, probability, and DI sets in table, bar, pie, or caselet form.
Core rules
- Percentage change is always on the original base: a 20% rise then a 20% fall gives 0.8 × 1.2 = 0.96, a 4% net fall.
- For compound interest, use A = P(1 + r/100)^n; for simple interest, I = PRT/100. Half-yearly compounding halves the rate and doubles the periods.
- Work problems: convert days to rates. If A does a job in 12 days and B in 18 days, their rates are 1/12 and 1/18; combined rate = 5/36, so time = 36/5 = 7.2 days.
- In DI, read the units before calculating and use approximation when options are far apart.
Worked example
A sum becomes ₹13,310 in 3 years at 10% per annum compound interest. Find the principal. A = P(1.1)^3 = 1.331P. So P = 13,310 / 1.331 = ₹10,000. Check: 10,000 × 1.1 = 11,000; × 1.1 = 12,100; × 1.1 = 13,310.
Traps
Applying a percentage to the changed value rather than the original; mixing simple and compound interest formulas; over-calculating in 'approximate' questions and then picking a nearby wrong option; and in quadratic equations, reversing the sign relation between roots and coefficients.