Answer
please see details in "work step by step"
Work Step by Step
Exponential growth model: $A=A_{0}e^{kt} \qquad(k>0)$
($A_{0}$ is the initial quantity, $A$ is the quantity after time t).
We solve for t (the time it takes for $A$ to become $2A_{0}$).
$2 A_{0}=A_{0}e^{kt}\displaystyle \qquad .../\times\frac{1}{A_{o}}$
$2=e^{kt}\qquad .../$ take ln( ) of both sides
$\ln 2=kt$
$t=\displaystyle \frac{\ln 2}{k}$
The population doubles in $t=\displaystyle \frac{\ln 2}{k}$ years,
which confirms the problem statement.