Quantitative Aptitude and Data Interpretation
What this section tests
Quantitative Aptitude and Data Interpretation tests speed with accurate arithmetic plus the ability to read a table, bar, line, pie, radar or caselet and answer ratio, percentage and average questions from it. Recurring topics: simplification and approximation, number series (missing and wrong term), quadratic comparison, percentage, ratio-proportion, average, time-work and pipes, speed-distance, boats, SI/CI, profit-loss, mixtures and alligation, partnership, probability, permutations, mensuration.
Core rules
- Successive percentage change: net = a + b + ab/100. Two successive discounts of 10% and 5% give 14.5%, not 15%.
- Compound interest: A = P(1 + r/100)^n. The doubling period is constant, so money that doubles in 5 years becomes 4 times in 10 years and 8 times in 15.
- In time-work, add efficiencies (1/time), never the times themselves.
- In DI, convert every figure to one unit (thousand/lakh/crore) before dividing, and prefer ratio comparisons over computing large absolute values.
Worked example
An article costs ₹100. The shopkeeper marks it up 40%, then allows 10% and 5% off. Marked price = 140. After 10%: 126. After a further 5%: 126 × 0.95 = 119.70. Profit = ₹19.70 on ₹100 = 19.7%, even though the discounts look like 15%.
Traps
- Applying a discount to cost price instead of marked price.
- Treating the base of a percentage change wrongly: "A is 25% more than B" means A = 1.25B, so B is 20% less than A.
- Reading a DI table's "% of total" column when the question asks "% more than".
- Over-rounding in approximation questions before checking how far apart the options are.