A rod flies with constant velocity past a mark which is stationary in the reference frame \(K .\) In the frame \(K\) it takes \(\Delta t=\) \(=20\) ns for the rod to fly past the mark. In the reference frame fixed to the rod the mark moves past the rod for \(\Delta t^{\prime}=25 \mathrm{~ns}\). Find the proper length of the rod.