Aptitude and Logical Reasoning: Fast Methods and Answer Traps
What the aptitude and reasoning section tests
Quantitative aptitude covers percentages, ratio and proportion, averages, profit and loss, simple and compound interest, time–speed–distance, time and work, permutations and combinations, and probability. Logical reasoning covers series completion, coding–decoding, blood relations, direction sense, syllogisms, and seating arrangement.
Core rules
- Percentage change = (new − old)/old × 100; divide by the original value.
- Successive changes of a% and b% give a net change of a + b + ab/100, not a + b.
- Speed = distance/time; convert km/h to m/s by multiplying by 5/18.
- If A finishes a job in x days, A's one-day work is 1/x; combined rates add.
- In syllogisms, "all A are B" does not imply "all B are A". "Some A are B" is symmetric, but subject–predicate order still matters in conclusions.
- In blood relations, draw a family tree and mark gender before answering.
Worked example
A shopkeeper marks an item 40% above cost and offers a 25% discount. Find the profit percentage. Let cost = ₹100. Marked price = ₹140. Selling price = 140 × 0.75 = ₹105. Profit = ₹5 on a cost of ₹100, so profit = 5%.
Traps that cost marks
- Adding successive percentage changes instead of compounding them.
- Taking the discount on the selling price rather than the marked price.
- Converting "some" statements into "all" conclusions.
- Losing track of facing direction after turns in direction-sense problems.
- Rounding intermediate fractions in ratio questions; keep fractions until the final step.
- Ignoring that in compound interest the base grows each period.