Let $E=\\{x_i\\}$ be an indexation for the numerable set $E$, then $E=\bigcup \\{x_n\\}$. Since $m^*(\\{x_n\\})=0$ for each $n\in\mathbb{N}$ (is easy to see that a single point set has outer measure $0$), then by this question $m^*(E)=0$.
Let $E=\\{x_i\\}$ be an indexation for the numerable set $E$, then $E=\bigcup \\{x_n\\}$. Since $m^*(\\{x_n\\})=0$ for each $n\in\mathbb{N}$ (is easy to see that a single point set has outer measure $0$), then by this question $m^*(E)=0$.