Quantitative Aptitude and Numerical Ability
Quantitative Aptitude in the LDC paper is speed-based: simplification, number system, percentage, ratio-proportion, average, profit and loss, simple and compound interest, time-work, time-speed-distance, mensuration and data interpretation.
Core shortcuts
- Successive percentage change: a + b + ab/100 (signs matter).
- Profit% = (SP − CP)/CP × 100; discount is always calculated on marked price.
- SI = PRT/100; CI = P(1 + r/100)^n − P.
- Work: one day's work = 1/A + 1/B; combined time = AB/(A + B).
- km/h to m/s: multiply by 5/18.
- Average = sum ÷ count; adding a value above the average raises it by (value − average)/new count.
Worked example
A finishes a job in 12 days, B in 18 days. Together, one day's work = 1/12 + 1/18 = 3/36 + 2/36 = 5/36. Time = 36/5 = 7.2 days. Reasoning: add rates, never times. Adding 12 + 18 = 30 and halving would be wrong because the two workers do not work at the same speed.
Traps that cost marks
- Computing percentage on the wrong base - profit% is on CP, discount% on MP.
- Assuming CI equals SI in year one: true for year one, false afterwards.
- Mixing units (metres with km/h, minutes with hours).
- Averaging averages as if all groups had equal size.
- Rounding early; carry fractions until the final step.
Method
Convert to a single unit, write the formula, substitute, then estimate the answer to check the magnitude.