General Arithmetic
What this domain tests
General Arithmetic tests numerical accuracy and speed in number system, percentages, ratio and proportion, averages, profit and loss, simple and compound interest, time–work, speed–distance–time, and mensuration. Most questions are direct if you set up the correct relation.
Core rules
- Follow BODMAS: brackets, orders, division/multiplication, addition/subtraction.
- Percentage change: increase = (change ÷ original) × 100. Successive changes multiply: a 20% rise then 10% fall gives 1.20 × 0.90 = 1.08, i.e. 8% net rise.
- Speed conversion: km/h × 5/18 = m/s; m/s × 18/5 = km/h.
- Time and work: if A finishes in x days and B in y days, together they finish in xy/(x + y) days.
- Simple vs compound interest: SI = PRT/100; CI = P[(1 + R/100)^T − 1].
- Mensuration: area of circle = πr²; volume of cylinder = πr²h; use consistent units.
Worked example
A train 200 m long runs at 72 km/h and crosses a platform in 20 s. Find the platform length. Reasoning: 72 km/h = 72 × 5/18 = 20 m/s. Total distance covered in 20 s = 20 × 20 = 400 m. This equals train length + platform length, so platform = 400 − 200 = 200 m. The trap is using only the train length or forgetting to convert km/h.
Traps that cost marks
Mixing SI and CI formulas; applying successive percentage changes additively; ignoring unit conversions; and calculating “time to cross a platform” as if only the train length mattered. Estimate the answer first, then compute exactly.