Mathematics / Quantitative Aptitude
Mathematics is arithmetic-heavy: number system, percentage, ratio and proportion, average, profit and loss, simple and compound interest, time–speed–distance, time and work, mensuration, and data interpretation.
Rules worth memorising
- Percentage change is always calculated on the original (base) value.
- Profit and loss are computed on cost price unless stated otherwise; discount is always on marked price.
- Successive change of a% and b% gives a net change of a + b + ab/100.
- Simple interest = PRT/100; compound interest = P(1 + R/100)^T − P.
- Time and work: convert days into work done per day; combined rate is the sum of individual rates.
- Speed conversion: 1 km/h = 5/18 m/s.
Worked example
A trader marks an article 40% above cost and then allows a 25% discount. What is his profit? Take cost price = 100, so marked price = 140. Selling price = 140 × 75/100 = 105. Profit = 105 − 100 = 5, which is 5%. The discount is applied to the marked price while profit is measured against the cost price - the two bases differ, which is exactly what the question tests.
Traps
Applying the discount to cost price; assuming a 25% mark-up and a 25% discount cancel out; forgetting that compound interest exceeds simple interest from the second year; and losing marks to unit mismatch (minutes versus hours, centimetres versus metres). In data interpretation, read the axis labels and units before computing anything.