Answer
\begin{equation}
\mu=\frac{\ln 2}{20}
\end{equation}
also, see the following image:
Work Step by Step
Solve the simple given equation to obtain
$$
N(t)=N_{0} e^{\mu t}
$$
where $N_{0}=N(0)$
In order to determine $\mu$ , use the given information $N(20)=2 N(0)$
\begin{array}{c}{N_{0} e^{20 \mu}=2 \cdot N_{0} e^{0 \mu}} \\ {\Leftrightarrow e^{20 \mu}=2} \\ {\Leftrightarrow 20 \mu=\ln 2}\end{array}
\begin{equation}
\mu=\frac{\ln 2}{20}
\end{equation}
