Quantitative Aptitude / Data Analysis & Interpretation
What to master
Simplification and approximation, number series (missing and wrong term), quadratic equations and comparison, and the arithmetic core: percentage, ratio and proportion, average, profit and loss, simple and compound interest, time-speed-distance including trains and boats, pipes and cisterns, mixtures and alligation, work, and mensuration. DI appears as tabular, bar, line, pie, caselet, radar and missing-value sets, with a few probability and permutation questions embedded.
Traps
- Percentage change: always identify the base. A price up 20% then down 20% does not return to the original, because the second change applies to the new value.
- Successive discounts are applied one after the other, not added. 20% then 10% = 1 − 0.8 × 0.9 = 28%, not 30%.
- Half-yearly compounding halves the rate and doubles the periods.
- Average speed over equal distances is the harmonic mean 2xy/(x+y), not (x+y)/2.
- In DI, 'percent of total' versus 'percentage points', and units in lakh or crore, cause the commonest slips. Read headers and footnotes.
- In approximation, check how far apart the options are before rounding aggressively.
Rule of thumb
For every percentage question, write the base beside the number before computing. In DI, do one quick pass to mark which questions need exact work and which can be estimated.
Worked example
A shopkeeper marks up by 40% and offers 25% discount. Net factor = 1.4 × 0.75 = 1.05, so a 5% profit, not 15%.