Quantitative Aptitude
Quantitative Aptitude tests numerical fluency under time pressure: simplification, percentages, ratio and proportion, averages, simple and compound interest, time-work, speed-distance, mensuration and data interpretation. The safest method is to convert every statement into one equation or multiplier before calculating.
Core rules
- Percentage change = (change ÷ original) × 100; use the original as the base, not the new value.
- A 20% discount means multiply by 0.80; a 25% profit means selling price = 1.25 × cost price.
- Simple interest = P × R × T ÷ 100, with T in years. Compound interest = P(1 + R/100)^T − P.
- Work rates add: if A finishes in 12 days and B in 18 days, together they do 1/12 + 1/18 = 5/36 per day, so 36/5 = 7.2 days.
- In speed problems, keep units consistent; convert km/h to m/s by multiplying by 5/18.
Worked example
A shop marks an item at ₹800, gives a 20% discount, and still makes 25% profit. Find the cost price. Selling price = 800 × 0.80 = ₹640. Since SP = 1.25 × CP, CP = 640 ÷ 1.25 = ₹512. Check: profit = 640 − 512 = ₹128, and 128/512 = 25%.
Traps
Do not add successive percentage changes (20% then 10% is 32%, not 30%). Do not use the selling price as the base for a discount. In CI, do not apply simple interest for the second year. In DI, read whether the question asks for percentage of total or percentage change.