Quantitative Aptitude and Numerical Ability
The Quantitative Aptitude and Numerical Ability section checks speed and accuracy on school-level arithmetic. THDC JE papers favour percentage, ratio, average, time–speed–distance, time and work, simple and compound interest, mensuration and data interpretation. There is no calculator, so choose the shortest valid method and estimate before computing.
Rules that carry most questions
- Percentage change = (new − old)/old × 100; successive changes multiply, not add: +20% then −10% gives 1.2 × 0.9 = 1.08, i.e. +8%.
- Average speed = total distance / total time, never the arithmetic mean of speeds unless times are equal.
- Work: if A finishes in x days, one day's work = 1/x; combined rate = 1/x + 1/y.
- CI: A = P(1 + r/100)^n; SI = PRT/100; the CI−SI gap for 2 years is P(r/100)².
- Mensuration: circle area = πr², circumference = 2πr; cylinder volume = πr²h.
Worked example
A 150 m train crosses a 300 m bridge in 30 s. Find its speed in km/h. Total distance = 150 + 300 = 450 m, because the train must clear the bridge plus its own length. Speed = 450/30 = 15 m/s. Convert: 15 × 18/5 = 54 km/h.
Traps
Using only the train length; multiplying by 18/5 when converting km/h to m/s; averaging two speeds; adding successive discounts instead of multiplying; forgetting to convert paise, minutes or hours; and applying an annual rate to half-yearly compounding without halving it.