Arithmetic and Numerical Ability
What the paper tests
Speed and accuracy on school-level arithmetic: percentages, ratio and proportion, averages, profit and loss, simple and compound interest, time and work, time and distance, LCM and HCF, simplification and number series, and basic mensuration.
Core rules
- Percentage change: increase of x% then decrease of x% always leaves a net loss of x²/100 per cent.
- Simple interest: SI = P × R × T / 100. If a sum doubles, SI earned equals P, so R × T = 100.
- Compound interest: A = P(1 + R/100)^T; CI = A − P.
- Time and work: convert days to work-per-day (1/n). Combined rate is the sum of individual rates; time = 1/(sum of rates).
- Time and distance: speed = distance/time. For a train passing a pole, distance = length of the train; passing a platform, distance = length + platform.
- Ratio: if a : b = 2 : 3 and b : c = 4 : 5, make b common (8) to get a : b : c = 8 : 12 : 15.
- Averages: total = average × number of items; adding a value shifts the average by the deviation divided by the new count.
Worked example
A sum of money doubles itself in 8 years at simple interest. Find the rate per annum. Reasoning: doubling means the interest earned equals the principal, so SI = P. Then P = P × R × 8 / 100, giving R × 8 = 100, so R = 12.5% per annum. A candidate who assumes compound interest will get a different, wrong figure - read whether the question says simple or compound.
Traps
Forgetting to convert months into years in the interest formula; treating the discount percentage and profit percentage as if they were computed on the same base; and in time-and-work problems, subtracting rates when a worker leaves instead of adding them.