The revenue function is:
[tex]R(x)=x\cdot p(x)[/tex]
where p(x) is given by:
[tex]p(x)=27(5)^{\frac{-x}{6}}[/tex]
For x=18, you obtain for p(x):
[tex]p(18)=27(5)^{-\frac{18}{6}}=27(5)^{-3}=\frac{27}{125}[/tex]
Then, for R(x):
[tex]R(18)=18\cdot p(18)=18\cdot\frac{27}{125}\approx3.89[/tex]
Then, if 18 units are sold the revenue is approximately 3.89 dollars.
