Quantitative Aptitude and Reasoning
Quantitative Aptitude and Reasoning tests arithmetic, percentages, ratio, averages, time-work, speed-distance, interest, series, coding-decoding, syllogism, blood relations and directions. Speed with the correct method matters more than long calculation.
Core rules
- Percentage change: an increase of r% followed by a decrease of r% gives a net decrease of r squared divided by 100 percent.
- Average = sum divided by count. For equal distances at speeds x and y, average speed = 2xy/(x+y), not (x+y)/2.
- Time-work: rate = 1/days. If A alone takes 10 days and B alone takes 15 days, together the rate is 1/10 + 1/15 = 1/6, so they finish in 6 days.
- Ratio: convert quantities to the same unit; chain ratios by multiplying corresponding terms.
- Reasoning: check differences, ratios, squares, cubes and alternating patterns in series; draw Venn diagrams for syllogism; track clockwise and anticlockwise turns for directions.
Worked example
A price is increased by 20% and then reduced by 20%. Start with 100: 100 becomes 120, then 120 becomes 96, a net 4% decrease. Formula: 20 - 20 - (20 x 20 / 100) = -4%. Reasoning: the second 20% is calculated on the higher amount, so it is larger in rupees.
Traps
Successive discounts are not additive. Average speed is not the arithmetic mean when distances are equal. In blood relations, maternal uncle is the mother's brother. In syllogism, some A are B does not imply some B are not A unless stated.