Numerical Ability / Quantitative Aptitude
What Numerical Ability tests
The paper tests calculation speed rather than advanced mathematics: number system, percentages, ratio and proportion, averages, profit and loss, simple and compound interest, time–work, time–speed–distance, mensuration and data interpretation.
Formulas to keep at hand
- Percentage change = (change ÷ original) × 100; always identify the base first.
- Successive changes of a% and b% give a net change of a + b + (ab ÷ 100) per cent.
- SI = PRT ÷ 100; CI = P(1 + r/100)^n − P. For two years, CI − SI = P(r/100)².
- Work: convert days into units per day by taking the LCM of the individual times.
- Speed conversion: km/h × 5/18 = m/s.
- Average = sum ÷ count; a new member's value = new average + (change in average × new count).
Worked example
Two constables can complete a verification drive in 12 and 18 days working alone. How long do they take together? Reasoning: the LCM of 12 and 18 is 36 units of work. The first does 3 units per day, the second 2 units per day, together 5 units per day. 36 ÷ 5 = 7.2 days. Never add rates as fractions without a common denominator.
Traps that cost marks
- A is 25% more than B means A = 1.25B; the reverse is not 25% less but 20% less.
- Successive discounts of 10% and 20% equal 28% off, not 30%.
- Mensuration answers must match units - convert cm to m before computing area or volume.
- In data interpretation, read the axis scale and the year column; judging by bar height alone misleads.