ANSWER:
122 gallons
STEP-BY-STEP EXPLANATION:
We have the following function:
[tex]V=55\cdot\: cos\mleft(\frac{2\pi}{7}t\mright)+67[/tex]
The first thing is to calculate the periodicity of the function. The value of the periodicity would be the value of x (in this case t) to calculate the value of y, that is, in this case the volume.
As follows:
[tex]\begin{gathered} p=\frac{2\pi}{\frac{2\pi}{7}}=7 \\ \text{therefore x = 7, replacing} \\ V=55\cdot\: cos(\frac{2\pi}{7}\cdot7)+67 \\ V=55\cdot\: cos(2\pi)+67 \\ V=55+67 \\ V=122 \end{gathered}[/tex]
Therefore the maximum volume would be 122 gallons.