we know 12 gallons is 5/6, so there was some in the car already, the whole tank will be 6/6 or one whole.
if 12 is 5/6, how much is 6/6, or the whole thing?
[tex]\bf \begin{array}{ccll}
gallons&tan\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
12&\frac{5}{6}\\\\
g&\frac{6}{6}
\end{array}\implies \cfrac{12}{g}=\cfrac{\quad \frac{5}{6}\quad }{\frac{6}{6}}\implies \cfrac{12}{g}=\cfrac{5}{6}\cdot \cfrac{6}{6}\implies \cfrac{12}{g}=\cfrac{5}{6}
\\\\\\
72=5g\implies \cfrac{72}{5}=g\implies 14\frac{2}{5}=g[/tex]
