Eduqo
Quantitative Aptitude›Ch 2: Commercial Maths›Percentage Mastery
Master fractions, successive change, and population growth for SSC & Defence exams.
A percentage is simply a fraction with a denominator of 100. Converting fractions to percentages and applying net successive change saves crucial time in exams.
Memory Hooks
| Fraction | Percentage | Application |
|---|---|---|
| 1/2 | 50% | Half |
| 1/3 | 33.33% | 1/3rd |
| 1/4 | 25% | Quarter |
| 1/8 | 12.5% | Fast calc |
Successive
(Net percentage change)
Net percentage change
+10% then +20% equals +32% net growth.
Population
(Compound growth model)
Compound growth model
Grows year over year like compound interest.
Consumption
(Inverse price relation)
Inverse price relation
Price rise forces consumption cut to keep budget fixed.
Multiplier
(Direct scaling factor)
Direct scaling factor
Multiply by 1.15 to instantly add a 15% increase.
Add 10% and 20% to get 30% netUse formula: 10 + 20 + (200/100) = 32%
Successive changes never add up linearly; always account for the base shift.
Price up 20% means consumption down 20%Consumption drops by 20/120 = 16.67%
Percentage decrease for consumption uses the new higher price base.
Population 3 years ago was Future/(1+r/100)^3Past Population = Present / (1 + r/100)^t
Be careful with time direction; past population calculation divides, future multiplies.
Q1.The price of sugar increases by 20%. By what percentage must a housewife reduce consumption to keep expenditure constant?
Q2.A town's population increases by 10% in the first year and 20% in the second year. What is the net percentage growth?