Quantitative Aptitude
Quantitative Aptitude covers arithmetic and elementary mensuration: percentage, ratio and proportion, average, profit and loss, simple and compound interest, time and work, pipes and cisterns, time–speed–distance, mixtures, mensuration, permutations and combinations, probability, and data interpretation. The questions are deliberately short, so accuracy under a two-minute-per-question rhythm matters more than advanced technique.
Core rules
- Percentage change is always measured on the original base.
- Successive changes multiply: +20% then −20% gives 0.96, a 4% net loss, not zero.
- Profit or loss percentage is on cost price unless the question states selling price.
- Compound interest: CI = P(1 + r/100)^n − P. For two years, CI − SI = P(r/100)².
- Time and work: convert to rates, or take total work as the LCM of the days given.
- Speed: km/h × 5/18 = m/s. For equal distances, average speed is 2xy/(x+y), never (x+y)/2.
- Mensuration: circle area πr², cylinder πr²h, sphere (4/3)πr³, cone (1/3)πr²h.
Worked example
Two pipes fill a tank in 12 minutes and 18 minutes; a drain empties it in 36 minutes. All are opened together - how long to fill? Take tank capacity as 36 units, the LCM. Pipe A gives 3 units per minute, pipe B gives 2, the drain removes 1. Net = 4 units per minute. Time = 36 ÷ 4 = 9 minutes. Fraction check: 1/12 + 1/18 − 1/36 = 3/36 + 2/36 − 1/36 = 4/36 = 1/9, so 9 minutes. Agreement of both methods is the confirmation.
Traps
- Adding successive discounts: 10% then 20% is 28%, not 30%.
- Averaging two speeds arithmetically instead of harmonically.
- Mixing profit percentage calculated on cost price with one calculated on selling price.
- Skipping unit conversion in speed problems or mishandling leap days in age problems.
- Rounding too early in data interpretation sets.