Quantitative Aptitude and Logical Reasoning: Method Over Speed
What to master
Quantitative items cover percentages, ratios, averages, profit and loss, simple and compound interest, time-speed-distance, work rates, and basic algebra and geometry. Reasoning items cover series, analogies, coding-decoding, syllogisms, blood relations, direction sense, and seating arrangement.
Quantitative habits
Convert word problems into a single equation before computing. Use benchmarks: 12.5 percent is one-eighth, 16.67 percent is one-sixth, 25 percent is one-quarter. In percentage-change problems, apply successive changes multiplicatively, not additively. For work problems, assign total work a convenient value such as the least common multiple of the given days. Estimate first so that an implausible answer is caught immediately.
Reasoning habits
Draw the arrangement or the family tree; never hold relations in your head. For syllogisms, test the conclusion against a counterexample diagram - if one diagram makes the premises true and the conclusion false, the conclusion does not follow. For series, check differences, then ratios, then alternating patterns.
Traps
- Percentage increase then decrease of the same rate does not return to the original value.
- Average speed over equal distances is the harmonic mean, not the arithmetic mean.
- In seating puzzles, "left" and "right" depend on facing direction.
- In syllogisms, "some A are B" does not imply "some B are A" in every formal system unless conversion is permitted - check the convention stated.
Worked example
A price rises 20 percent then falls 20 percent. Multipliers are 1.2 and 0.8, product 0.96, so the net result is a 4 percent decrease. Adding and subtracting 20 would wrongly give zero.