Arithmetical & Numerical Ability
Arithmetical & Numerical Ability tests school-level arithmetic under speed pressure: simplification and BODMAS, percentages, ratio and proportion, averages, profit and loss, simple and compound interest, time–work, pipes and cisterns, speed–distance–time, mensuration, and data interpretation. The maths is not advanced; the marks are lost through unit slips and avoidable calculation.
Core rules
- Percentage: always identify the base; a 20% increase followed by a 20% decrease gives 96%, not 100%.
- Profit/loss: use cost price as the base unless the question explicitly says otherwise.
- Interest: SI = PRT/100; CI = P(1+r/100)^t − P. For two years, CI − SI = P(r/100)^2.
- Time–work: convert days into rates or take the LCM of days as total work units.
- Average speed: for equal distances at speeds x and y, average speed = 2xy/(x+y), not (x+y)/2.
Worked example
A completes a job in 12 days and B in 18 days. They work together for 4 days, then A leaves. How long does B take to finish? Take total work = LCM(12, 18) = 36 units. A does 3 units/day, B does 2 units/day. Together they do 5 units/day, so in 4 days they finish 20 units. Remaining = 16 units. B alone takes 16 ÷ 2 = 8 days. Answer: 8 days.
Traps
- Using the wrong base in successive percentage change.
- Mixing SI and CI formulas for two-year differences.
- Forgetting to convert minutes to hours in speed questions.
- Using diameter instead of radius in area and volume questions.