Quantitative Aptitude and Numerical Ability
This domain tests speed and accuracy in arithmetic: number system, percentage, ratio and proportion, average, profit and loss, simple and compound interest, time-work, time-speed-distance, mensuration and simplification using BODMAS.
Core rules to memorise
- Percentage-to-fraction: 1/8 = 12.5%, 1/6 = 16.66%, 1/3 = 33.33%, 1/16 = 6.25%.
- Average = sum ÷ number of items; if every value rises by k, the average rises by k.
- Profit% and loss% are always calculated on cost price unless stated otherwise.
- Successive changes multiply: two successive changes of a% and b% give a net change of a + b + ab/100.
- Simple interest = P × R × T / 100; compound interest compounds each year.
Worked example
A worker's honorarium is increased by 20% and then reduced by 20%. What is the net change? Reasoning: never subtract the percentages. Take the original as 100. After a 20% rise it becomes 120. A 20% cut applies to 120, not to 100, so the cut is 24, leaving 96. Net change = 96 − 100 = −4, i.e. a 4% decrease. Using the formula: 20 + (−20) + (20 × −20)/100 = −4%.
Traps that cost marks
Adding successive percentages instead of multiplying them; applying loss percentage on selling price; confusing percentage points with percent; averaging averages without weighting by group size; and forgetting unit conversion in mensuration (1 m = 100 cm, so area converts by 10,000).