Question

Suppose a population is growing by 8% each year. How many years will it take the population to double?

Answer 1

ANSWER

$\text{9 years}$

EXPLANATION

We want to find the number of years that it will take the population to double.

To do this, we have to apply the exponential growth function:

$y=a(1+r)^t$

where y = final value

a = initial value

r = rate of growth

t = time (in years)

For the population to double, it means that the final value must be 2 times the initial value:

$y=2a$

Substitute the given values into the function above:

$\begin{gathered} 2a=a(1+\frac{8}{100})^t \\ \frac{2a}{a}=(1+0.08)^t=1.08^t \\ 2=1.08^t \end{gathered}$

To solve further, convert the function from an exponential function to a logarithmic function as follows:

$\log _{1.08}2=t$

Solve for t:

$\begin{gathered} \frac{\log _{10}2}{\log _{10}1.08}=t \\ \Rightarrow t=9\text{ years} \end{gathered}$

It will take 9 years for the population to double.