Given data:
The given distance is d=480 feet.
The given time is t=24 seconds.
The speed of the submarine is,
[tex]\begin{gathered} s=\frac{d}{t} \\ s=\frac{480\text{ ft}}{24\text{ s}} \\ =20\text{ ft/s} \end{gathered}[/tex]The expression for the change in elevation after 8 seconds is,
[tex]\begin{gathered} D=s\times(8\sec ) \\ =(20\text{ ft/s)(8 s)} \\ =160\text{ ft} \end{gathered}[/tex]Thus, the change in elevation after 8 seconds is 160 ft.