Discussion
What will be difference in population 3 years ago and 2 years ago of Devon village, whose current population is 100000 and which is increasing at a rate of 25% every year?
Answer: D
The population grows at 25% a year, so past figures are found by dividing, not subtracting.
\[ P_{-3} = \frac{100000}{(1.25)^{3}} = \frac{100000}{1.953125} = 51{,}200 \]
\[ P_{-2} = \frac{100000}{(1.25)^{2}} = \frac{100000}{1.5625} = 64{,}000 \]
\[ \text{difference} = 64000 - 51200 = 12{,}800 \]
Reducing by 25% a year instead of dividing by 1.25 would give 42,187 and 56,250 - a quite different pair. Growth of 25% is undone by a cut of 20%, not 25%.
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