Shortcut Trick
Net percentage change = x − y − (xy/100) = 10 − 10 − (10 × 10)/100 = −1%.
Successive change of 10% increase and 10% decrease results in a net 1% decrease.
99% of initial population = 4,73,220.
100% of initial population = (4,73,220 ÷ 99) × 100 = 4,78,000.
∴ The correct answer is 4,78,000.
Alternate Method
Given:
First year increase = 10%
Second year decrease = 10%
Final population = 4,73,220
Let the population at the beginning of the first year be P.
⇒ P × (1 + 10/100) × (1 − 10/100) = 4,73,220
⇒ P × (110/100) × (90/100) = 4,73,220
⇒ P × (11/10) × (9/10) = 4,73,220
⇒ P × (99/100) = 4,73,220
⇒ P = (4,73,220 × 100) ÷ 99
⇒ P = 4,780 × 100
⇒ P = 4,78,000
∴ The correct answer is 4,78,000.
Additional Information
Successive Percentage Change
When a value changes by x% and then y%, the net change is given by [x + y + (xy/100)]%.
Same Percent Increase and Decrease
If a value is increased by x% and then decreased by x%, there is always a net decrease of (x2/100)%.
Population Formula
Population after n years = P(1 ± R/100)n, where P is the initial population and R is the rate of growth/decay.