Explanation: \begin{equation} m_{0}c^{2}=mc^{2}-m_{0}c^{2} \end{equation} \begin{equation} 2m_{0}c^{2}=mc^{2} \end{equation} \begin{equation} 2m_{0}=m \end{equation} \begin{equation} 2m_{0}=\frac{m_{0}}{\sqrt{1-\frac{v^{2}}{c^{2}}}} \end{equation} \begin{equation} 4=\frac{1}{1-\frac{v^{2}}{c^{2}}} \end{equation} \begin{equation} 1-\frac{v^{2}}{c^{2}}=\frac{1}{4} \end{equation} \begin{equation} \frac{v^{2}}{c^{2}}=1-\frac{1}{4}=\frac{3}{4} \end{equation} \begin{equation} v=\frac{\sqrt{3}}{2}c \end{equation} \begin{equation} v=0.866~c \end{equation}