Arithmetic and Numerical Ability
This domain 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, simplification, number series, elementary mensuration and data interpretation. Questions are short but deliberately designed so that a candidate who computes in the wrong order loses two minutes.
Core rules
- Percentage change = (change / original) × 100. Two successive changes of a% and b% give a net change of a + b + ab/100.
- Time and work: convert days into a per-day rate, then add rates. If A finishes in x days and B in y days, together they finish in xy/(x+y) days.
- Simple interest = PRT/100; compound interest compounds each period, so do not add simple interest for more than one year.
- Speed: distance = speed × time; convert km/h to m/s by multiplying by 5/18.
Worked example
A can complete a file-sorting task in 12 days and B in 18 days. Working together, how long will they take? Reasoning: take the LCM of 12 and 18, which is 36 units of work. A does 3 units a day, B does 2 units a day; together 5 units a day. Time = 36 ÷ 5 = 7.2 days, i.e. 7⅕ days. The rate method avoids fractions until the final step and is far less error-prone than 1/12 + 1/18.
Traps that cost marks
Confusing percentage with percentage points; using the wrong base in "by what per cent did it increase"; forgetting that compound interest compounds; mixing ratio order (A:B versus B:A); failing to convert hours and minutes into the same unit; and leaving the answer un-simplified where options are given as fractions.