A vector $\overrightarrow{Q}$ which has a magnitude of 8 is added to the vector $\overrightarrow{P}$ which lies along the X-axis. The resultant of these two vectors is a third vector $\overrightarrow{R}$ which lies along the Y-axis and has a magnitude twice that of $\overrightarrow{P}$ . The magnitude of $\overrightarrow{P}$ is