Quantitative Aptitude
Quantitative Aptitude here rewards speed with school-level arithmetic, not advanced mathematics. Nearly every question comes from percentages, ratio and proportion, averages, time and work, speed and distance, profit and loss, simple and compound interest, mensuration and data interpretation. With a tight clock and negative marking, the deciding skill is picking the short method and knowing when to leave a question.
Rules worth memorising
- Fraction to percentage: 1/8 = 12.5%, 1/6 = 16.66%, 3/8 = 37.5%, 1/16 = 6.25%.
- Percentage change is always calculated on the original base, not the new value.
- Successive percentage changes of a and b give a net change of a + b + ab/100.
- Average speed over equal distances is 2xy/(x+y), not (x+y)/2.
- Simple interest is PRT/100; compound interest is P(1 + r/100)^t minus P.
- Convert km/h to m/s by multiplying by 5/18.
Worked example
A finishes a job in 12 days and B in 18 days. They work together for 3 days, then A leaves. How long does B need to finish the rest?
Take the LCM of 12 and 18, which is 36 units of work. A does 36/12 = 3 units a day, B does 36/18 = 2 units a day, together 5 units a day. In 3 days they complete 15 units, leaving 21. B alone finishes 21 units at 2 units a day, so 10.5 days. The LCM method avoids fractions throughout.
Traps that cost marks
- Adding successive percentages directly instead of using the ab/100 term.
- Treating average speed as the arithmetic mean of two speeds.
- Confusing simple and compound interest in two-year problems.
- Ignoring the changed total in ratio questions after a quantity is added or removed.
- Doing exact calculation in data interpretation when approximation is enough, and burning time on a question that should have been skipped.