Answer:
Every 5 seconds the beam rotates 90 degrees;
[tex]90^{\circ}[/tex]Explanation:
Given that the beam of a lighthouse makes one complete revolution every 20 seconds.
one complete revolution is;
[tex]360^{\circ^{}}[/tex]The rate of rotation is;
[tex]r=\frac{360^{\circ}}{20}=18^{\circ}\text{ per second}[/tex]The number of degrees it will rotate in 5 seconds is;
[tex]\begin{gathered} x=5\times r=5\times18^{\circ}per\text{ second} \\ x=90^{\circ} \end{gathered}[/tex]Therefore, every 5 seconds the beam rotates 90 degrees;
[tex]90^{\circ}[/tex]