Numerical Ability: Arithmetic Accuracy Under a Clock
Numerical Ability is school arithmetic against the clock. The topics are simplification, number series, percentage, average, ratio, profit and loss, simple interest, time and work, time-speed-distance, and basic mensuration. Getting the first ten seconds of each question right matters more than exotic methods.
Rules and shortcuts
- Learn fraction equivalents: 1/8 = 12.5%, 1/6 = 16.67%, 1/12 = 8.33%, 1/16 = 6.25%.
- Simple interest = PRT/100; amount = principal + interest.
- Profit and loss percentages are on cost price unless the question says otherwise: SP = CP × (100 + profit)/100.
- Work problems: take the LCM of the given days as total units, find each worker's per-day units, then add the rates.
- Convert km/h to m/s by multiplying by 5/18.
- Two successive discounts of a% and b% give one discount of a + b − ab/100, not a + b.
Worked example
A finishes a job in 12 days and B in 18 days. Working together, how long do they take? Reasoning: the LCM of 12 and 18 is 36, so take 36 units of work. A does 3 units a day, B does 2, together 5. Time = 36 ÷ 5 = 7.2 days, that is 7 days and about 4.8 hours. The LCM method keeps fractions out of the calculation entirely.
Traps
Adding percentages instead of compounding them; taking profit percent on selling price; leaving minutes and hours mixed in a speed question; recomputing an average after one value is removed without lowering the count; and rounding a clean fraction into a decimal before the final step.