Quantitative Aptitude and Reasoning
This domain measures speed with accuracy: arithmetic, its advanced applications, data interpretation, and logical reasoning. Cut-off here is usually decided by calculation speed, so method choice matters more than raw practice volume.
Core rules to have at your fingertips
- Percentage change is always computed on the original base; successive discounts of a% and b% equal a + b − ab/100.
- Simple interest is linear; compound interest grows geometrically - for two years, CI − SI = P(r/100)².
- Time and work: convert days to unit-per-day rates and add rates, never times.
- Ratio and proportion problems collapse when you assign a common multiplier k.
- Reasoning: syllogism follows the Venn method only; seating puzzles start from the most definite clue.
Worked example
A sum doubles in 5 years at simple interest. In how many years will it become four times itself? Reasoning: doubling means interest earned in 5 years equals P, so the rate is 20% per annum. To become four times, interest must equal 3P. At 20% per annum, 3P needs 3 × 5 = 15 years. Answer: 15 years. Note the linear structure of SI - this shortcut fails under compound interest.
Traps that cost marks
- Applying the SI doubling shortcut to compound interest questions.
- Percentage questions where the base shifts mid-problem (successive increase then decrease).
- Unit mismatch in speed-distance and in data interpretation (lakh versus crore, thousands versus millions).
- In syllogism, treating 'some A are B' as reversible without checking the conclusion's subject-predicate order.
- Over-calculating: many DI questions are answerable by approximation alone.