Single correct option
The population of a town increased by 20% in the first year, decreased by 25% in the second year and increased by 10% in the third year. If the population at the end of the third year was 39,600, what was the population at the start?
- A. 38,500
- B. 40,000
- C. 41,500
- D. 42,000
Show answer and solution
Answer: (B) 40,000
Net multiplying factor = 1.20 × 0.75 × 1.10 = 0.99.
Initial population = 39,600 / 0.99 = 40,000.
Note that a 20% rise followed by a 25% fall is not neutral: 1.2 × 0.75 = 0.9, a net 10% fall.
Tip: Work backwards by dividing by the product of the growth factors.
Initial population = 39,600 / 0.99 = 40,000.
Note that a 20% rise followed by a 25% fall is not neutral: 1.2 × 0.75 = 0.9, a net 10% fall.
Tip: Work backwards by dividing by the product of the growth factors.
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