Discussion
The population of a city is 50,000 at present. It increases at the rate of 10% per annum. What was its population 4 years ago from present?
Answer: C
Going backwards in time means dividing by the growth factor.
\[ P_{-4} = \frac{50000}{(1.1)^{4}} = \frac{50000}{1.4641} \]
\[ = 34{,}150.67 \approx 34{,}151 \]
Subtracting 10% four times instead would give \( 50000 \times 0.9^{4} = 32{,}805 \) - a different figure, because a 10% rise is undone by a fall of \( \tfrac{1}{11} \), not a fall of 10%.
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