Quantitative Aptitude: Speed Methods and Word Problems
Quantitative Aptitude
The section covers simplification and approximation, number series, quadratic equations, data interpretation and arithmetic word problems on percentage, ratio, time–work, speed–distance, interest, mixtures and probability. Speed comes from fraction conversion and estimation, not long division.
Core rules and concepts
- Follow BODMAS strictly; convert percentages to fractions: 12.5% = 1/8, 16⅔% = 1/6, 6.25% = 1/16, 37.5% = 3/8.
- Successive change: net change = a + b + ab/100.
- Interest: SI = PRT/100; the two-year gap between CI and SI is P(r/100)².
- Time and work: combined rate = 1/a + 1/b; take the LCM of days as total units of work.
- DI: read the heading and units before computing, and approximate when options are far apart.
Worked example
A sum doubles in 8 years at simple interest. In how many years will it become four times itself? Doubling means interest earned = P, so P = P × R × 8/100, giving R = 12.5%. For four times the sum, interest needed = 3P: 3P = P × 12.5 × T/100, so T = 300/12.5 = 24 years.
Traps that cost marks
- Using the two-year CI–SI difference formula when the question gives three years.
- Forgetting to halve the rate when interest compounds half-yearly.
- Confusing "more than" with "less than" in percentage-change questions.
- In DI, taking the wrong base year for a growth percentage, or ignoring a units mismatch (thousand vs lakh).