Numerical Ability / Quantitative Aptitude
Numerical Ability tests speed with correct method under a tight clock. The syllabus is arithmetic plus mensuration plus data interpretation: simplification, percentage, ratio and proportion, average, profit and loss, simple and compound interest, time–work and pipes, speed–time–distance including trains and boats, mixtures, areas and volumes, and table/bar/pie interpretation.
Rules that solve most questions
- Percentage change is always measured against the base stated, not the new value.
- Successive changes multiply: two successive discounts of 10% and 20% give 0.90 × 0.80 = 0.72, i.e. 28% off, not 30%.
- Time–work: take total work as the LCM of the individual days, convert each worker to units per day, then add rates.
- Trains: crossing a pole uses the train's own length; crossing a platform uses train length plus platform length. Convert km/h to m/s by multiplying by 5/18.
- Boats: downstream = boat + stream, upstream = boat − stream.
Worked example
A alone finishes a job in 12 days, B alone in 18 days. How long together? Reasoning: LCM of 12 and 18 is 36, so let total work = 36 units. A does 36/12 = 3 units/day; B does 36/18 = 2 units/day. Combined rate = 5 units/day. Time = 36/5 = 7.2 days, i.e. 7 1/5 days.
Traps
Applying a discount to the already-discounted price and calling it the total discount. Mixing simple and compound interest in the second year. Forgetting to add the train's own length in platform questions. Inverting upstream and downstream. Leaving an answer in the wrong unit (metres vs kilometres).