Quantitative Aptitude: Multipliers, Ratios and Data Interpretation
Quantitative Aptitude / Numerical Ability rewards clean arithmetic done quickly: simplification and approximation, number series, quadratic comparison, percentage, ratio and average, profit and loss, simple and compound interest, time and work, speed and distance, mixtures, mensuration, and data interpretation sets in table, bar, pie and caselet form. DI usually carries the largest single block of marks, so practise reading a chart and choosing the shortest path to the answer.
Think in multipliers, not additions
- A 25% increase is ×1.25; successive percentage changes multiply rather than add.
- Two successive discounts of a% and b% equal a single discount of a + b − ab/100.
- Compound interest: A = P(1 + r/100)^n. For two years, CI − SI = P(r/100)².
- A value raised and then lowered by the same x% always ends lower by x²/100 percent.
Ratio, alligation and work
- Alligation: (mean − cheaper)/(dearer − mean) gives the ratio of the two quantities mixed.
- In time-and-work, take the LCM of the individual days as total units of work, then convert each person to units per day.
- In speed problems, remember that time is inversely proportional to speed for a fixed distance.
Data interpretation
Read the title, the units and any footnote before calculating. When the options are widely spaced, round to two significant figures and move on.
Worked example
A shopkeeper marks an article 40% above cost and then gives successive discounts of 10% and 20%. Take CP = ₹100, so MP = ₹140. SP = 140 × 0.90 × 0.80 = ₹100.80, giving a profit of 0.8%. Adding the discounts to 30% gives SP = ₹98, a loss of 2% - that wrong value is deliberately placed among the options.
Traps that cost marks
- Applying a percentage change to the wrong base.
- Mixing up simple and compound interest after year one.
- Ignoring workers who join or leave mid-way in work problems.
- Solving a DI question exactly when approximation would have been enough.