Lets solve it in some more easier way
[tex]\\ \tt\longmapsto \dfrac{7mi}{\dfrac{1}{3}gal}[/tex]
[tex]\\ \tt\longmapsto\dfrac{7}{1/3}mi/gal[/tex]
[tex]\\ \tt\longmapsto 7\times 3[/tex]
[tex]\\ \tt\longmapsto 21mi/gal[/tex]
