Quantitative Aptitude / Numerical Ability
The Quantitative Aptitude section of the BoM Generalist Officer exam mixes speed arithmetic (simplification, approximation, number series, quadratic comparison) with applied arithmetic and one or two data-interpretation sets (table, bar, pie, caselet). Accuracy on DI and arithmetic carries more weight than raw calculation speed, because DI questions usually need two or three steps, not one.
Core rule bank
- Percentage: successive changes multiply, they do not add. A 20% rise then 20% fall leaves 96% of the original, a net 4% loss.
- Time and work: convert days to per-day rates and add rates, never days.
- Simple vs compound interest: CI compounded half-yearly means rate/2 and double the periods.
- Number series: test first differences, then second differences, then ratios, then squares/cubes with a constant offset.
- Data interpretation: percentage change = (new - old)/old x 100, always with the older value as base.
Worked example. A finishes a job in 12 days and B in 18 days. With C, the job finishes in 4 days. Find C's solo time. A's rate = 1/12, B's = 1/18, sum = 5/36 per day. Combined rate = 1/4 = 9/36. So C = 9/36 - 5/36 = 4/36 = 1/9, meaning C alone takes 9 days.
Traps that cost marks
Mixing percentage points with percent change; treating a discount followed by a markup as additive; using the wrong base year in DI ratio questions; forgetting unit conversion in speed-distance problems (km/h to m/s is x 5/18); and rounding too early in approximation questions, where the options sit close together. In quadratic comparison, check the sign convention before comparing roots.