ExplanationEvery 3 minutes, the population increases by 20% (which is the same as multiplying by 1.2).

Beginning at 8:54am, this change would occur at 8:57am and again at 9:00am. Use the variable x to represent the original quantity. Note that the 20% increase occurs twice:

x(1.2)(1.2) = 144,000

x = 100,000

Note that you cannot just reduce 144,000 by 20% twice, because 20% is not a percent of 144,000—it is a percent of the unknown, original number.

Alternatively, begin from 144,000 and calculate “backwards”:

From 8:57am to 9:00am: y(1.2) = 144,000, so y =\(\frac{144,000}{1.2}\) = 120,000.

From 8:54am to 8:57am: z(1.2) = 120,000, so z = \(\frac{120,000}{1.2}\)= 100,000.

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Sandy

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