Answer
$0.2 \pi \dfrac{rad}{sec} $
Work Step by Step
We know that the ships can see a blinking light every 5 seconds. There are 2 lights; hence a complete rotation takes 10 seconds.
Thus, the rotation speed $S$ can be computed as:
$S=\dfrac{1 \ rev }{10 \sec}=0.1 \dfrac{rev}{second} \\=\dfrac{0.1\cdot 2 \pi \ rad}{sec} \\=0.2 \pi \dfrac{rad}{sec} $
