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Suppose the population of a country in 1985 was 145 million. In 1995, it was 190 million. What is the population of the country in 2005?

A. [tex]P = 145 e^{(0.027)(10)}[/tex]
B. [tex]P = 145 e^{(0.027)(20)}[/tex]
C. [tex]P = 190 e^{(0.027)(10)}[/tex]
D. [tex]P = 190 e^{(0.027)(20)}[/tex]

Answer :

We start by assuming that the population grows continuously according to the model

[tex]$$
P(t) = P_0 \, e^{rt},
$$[/tex]

where [tex]$P_0$[/tex] is the initial population, [tex]$r$[/tex] is the continuous growth rate, and [tex]$t$[/tex] is the number of years.

1. Determine the Growth Rate

The population in 1985 is given as [tex]$145$[/tex] million and in 1995 as [tex]$190$[/tex] million. Since the time between 1985 and 1995 is [tex]$10$[/tex] years, we can set up the equation

[tex]$$
190 = 145 \, e^{r \cdot 10}.
$$[/tex]

To solve for [tex]$r$[/tex], divide both sides by [tex]$145$[/tex]:

[tex]$$
\frac{190}{145} = e^{10r}.
$$[/tex]

Taking the natural logarithm of both sides gives

[tex]$$
\ln\left(\frac{190}{145}\right) = 10r.
$$[/tex]

Therefore, the growth rate [tex]$r$[/tex] is

[tex]$$
r = \frac{1}{10} \ln\left(\frac{190}{145}\right).
$$[/tex]

Calculating the value yields approximately

[tex]$$
r \approx 0.027 \quad \text{per year}.
$$[/tex]

2. Calculate the Population in 2005

The year 2005 is [tex]$10$[/tex] years after 1995. Using the population from 1995 ([tex]$190$[/tex] million) as the starting point, we calculate the population in 2005 by

[tex]$$
P(2005) = 190 \, e^{r \cdot 10}.
$$[/tex]

Substituting the approximate value for [tex]$r$[/tex], we get

[tex]$$
P(2005) = 190 \, e^{0.027 \cdot 10}.
$$[/tex]

Evaluating this expression gives an approximate population of about [tex]$248.97$[/tex] million.

3. Matching with the Options

Among the choices provided:

- Option A: [tex]$P=145 e^{(0.027)(10)}$[/tex]
- Option B: [tex]$P=145 e^{(0.027)(20)}$[/tex]
- Option C: [tex]$P=190 e^{(0.027)(10)}$[/tex]
- Option D: [tex]$P=190 e^{(0.027)(20)}$[/tex]

The correct expression that we derived is

[tex]$$
\boxed{190 \, e^{(0.027)(10)}},
$$[/tex]

which corresponds to Option C.

Thus, the correct answer is Option C.

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