Quantitative Aptitude and Analytical Reasoning: Clean Methods Under Time Pressure
Quantitative Aptitude and Analytical Reasoning rewards clean method: translate the words into rates or totals, work in fractions, and eliminate options before doing long arithmetic.
Fast rules
- Percentage change: successive changes of a% then b% give a + b + ab/100, not a + b.
- Time and work: convert everything to a rate per unit time; combined rate is the sum, or the difference when one pipe empties.
- Averages: total = average × count; for close values use the assumed-mean deviation method.
- Interest: simple = PRT/100; compound = P(1 + R/100)^T − P.
- Data interpretation: note the units and the base year before computing anything.
Worked example
Pipe A fills a tank in 12 hours; pipe B empties it in 20 hours. Both are open. Rate = 1/12 − 1/20 = (5 − 3)/60 = 2/60 = 1/30, so the tank fills in 30 hours. The LCM check: take 60 units of capacity, A adds 5 units per hour, B removes 3, net 2, and 60 ÷ 2 = 30 hours. The second route is faster and catches a slip in the fraction.
Reasoning items
- Syllogisms: draw the minimal Venn diagram and assume nothing beyond the premises.
- Seating arrangements: fix the most constrained person first, then place neighbours.
- Series: test differences, then ratios, then squares and cubes.
Traps that cost marks
- Adding percentage changes instead of compounding them.
- Using the wrong base in successive discounts.
- Reversing 'all A are B' into 'all B are A'.
- Forgetting to convert hours to minutes in rate problems.