Quantitative Aptitude: Arithmetic and Data Interpretation
Quantitative Aptitude tests arithmetic and data interpretation under time pressure: percentages, ratio and proportion, time and work, speed and distance, interest, averages, and DI from tables, bar charts and pie charts.
Rules that carry marks
- Percentage change is always computed on the original base. Successive changes of a% and b% equal a + b + ab/100.
- In ratio problems, convert every quantity into a common number of parts before solving.
- In time-and-work, take the LCM of the given days as total work in units; each worker's rate is total ÷ LCM.
- Average speed = total distance ÷ total time; never the mean of the speeds.
- For two years, CI − SI = P(r/100)².
Worked example
A can finish a job in 12 days and B in 18 days. Working together, how long do they take?
- Total work = LCM(12, 18) = 36 units.
- A does 36/12 = 3 units per day; B does 36/18 = 2 units per day.
- Combined rate = 5 units per day.
- Time = 36/5 = 7.2 days, i.e. 7 days and 4.8 hours. The reasoning: converting days into work units removes fractions and keeps the arithmetic clean, so the answer is exact rather than approximate.
Traps
- Answering 40% less when the question says A is 40% more than B - the base differs.
- Averaging 60 km/h and 40 km/h to get 50 km/h when the distances are equal; the correct average is 48 km/h.
- Reading a DI table in lakh when the question asks for crore.
- Rounding mid-calculation and then finding that no option matches.