Substitute x=30 to find the profit of washing 30 cars:
[tex]\begin{gathered} f(x)=8(x-2)-20 \\ \Rightarrow f(30)=8(30-2)-20 \\ =8(28)-20 \\ =224-20 \\ =204 \end{gathered}[/tex]
Therefore, the profit for washing 30 cars is:
[tex]204\text{ dollars}[/tex]
