Numerical Aptitude: Setup Discipline over Calculator Speed
What this domain tests
Numerical Aptitude examines school-level arithmetic: number system, LCM and HCF, percentages, ratio and proportion, averages, profit and loss, simple and compound interest, time and work, time-speed-distance, mensuration, and data interpretation from tables and bar charts. The arithmetic is rarely difficult; marks are lost in the setup, not the calculation.
Rules worth memorising
- Percentage change is always measured against the original base. A rise from 40 to 50 is 25%, not 10%.
- Successive changes multiply. Two discounts of 20% and 10% give 0.8 × 0.9 = 0.72, a net 28% discount, never 30%.
- Average speed over equal distances is the harmonic mean, 2xy/(x+y), not (x+y)/2.
- For work problems, convert days into units per day using the LCM of the given days.
- SI = PRT/100; CI = P(1 + R/100)^T − P. Compound interest exceeds simple interest by the interest earned on the first year's interest.
Worked example
A finishes a job in 12 days and B in 18 days. Working together, how long do they take? Take total work as LCM(12, 18) = 36 units. A completes 36/12 = 3 units a day; B completes 36/18 = 2 units a day. Together they do 5 units a day. Time = 36/5 = 7.2 days, that is, 7 days and 4 hours 48 minutes.
Traps that cost marks
Applying a discount and a markup to different bases; treating compound interest as simple interest for two years; ignoring unit mismatches such as metres with centimetres or minutes with hours; and reading the wrong row or column in a data interpretation table.