Shortcut Trick
Let the original population be 100%.
Successive percentage change: 1.30 × 0.85 = 1.105.
Equating with present population: 1.105 units = 11,050.
1 unit = 11,050 ÷ 1.105 = 10,000.
∴ The correct answer is 10,000.
Alternate Method
Given:
Present Population = 11,050
Increase in 1st year = 30%
Decrease in 2nd year = 15%
Let the population 2 years ago be P.
⇒ P × (1 + 30/100) × (1 − 15/100) = 11,050
⇒ P × (130/100) × (85/100) = 11,050
⇒ P × (13/10) × (17/20) = 11,050
⇒ P × (221/200) = 11,050
⇒ P = (11,050 × 200) ÷ 221
⇒ P = 50 × 200
⇒ P = 10,000
∴ The correct answer is 10,000.
Additional Information
Successive Percentage Change Formula
Net change = [a + b + (ab/100)]%, where a and b are the percentage changes (use negative sign for decrease).
Population Growth Formula
Final Population = P × (1 ± r1/100) × (1 ± r2/100), where P is the initial population and r is the rate of change.
Effective Ratio Method
Convert percentages to fractions: 30% increase = 13/10 and 15% decrease = 17/20. Multiply ratios to find the total change factor.