If the cost of one gallon of gasoline is $2.39, to find the price of 'g' gallons of gasoline, we just need to multiply the number of gallons (g) by the cost of one gallon ($2.39), and that will be the cost C.
So we have the following equation:
[tex]C=2.39g[/tex]It's also possible to find this equation using a rule of three:
[tex]\begin{gathered} 1\text{ gallon}\to\text{\$}2.39 \\ g\text{ gallons}\to C \\ \frac{1}{g}=\frac{2.39}{C} \\ C=2.39g \end{gathered}[/tex]