Three point particles $P\mathit{,}\mathit{ }Q\mathit{,}\mathit{ }R$ move in a circle of radius $r$ with different but constant speeds. They start moving at $t=0$ from their initial positions as shown in the figure. The angular velocities (in $\mathrm{rad} {\sec }^{-1}$) of $P\mathit{,}\mathit{ }Q$ and $R$ are $5\mathrm{\pi }, 2\mathrm{\pi }$ and $3\pi$ respectively, in the same sense. The time at which they all meet is: