Using the given assumptions, the average fuel consumption per year is ...
[tex]\displaystyle \left(1.20\cdot 10^{8}\,\text{cars}\right)\times\frac{1\,\text{gal}}{20\,\text{mi}}\times\frac{1.2\cdot 10^{4}\,\text{mi}}{1\,\text{yr}\cdot \text{car}}=7.2\cdot 10^{10}\,\frac{\text{gal}}{\text{yr}}[/tex]
If gasoline consumption were reduced to 25 mi/gal, this would become
[tex]\dfrac{1.44\cdot 10^{12}\,\text{gal}}{0.25\cdot 10^{2}\,\text{yr}}=5.76\cdot 10^{10}\,\dfrac{\text{gal}}{\text{yr}}[/tex]
The amount of gasoline saved is the difference of these two values,
[tex](7.20-5.76)\cdot 10^{10}\,\dfrac{\text{gal}}{\text{yr}}=1.44\cdot 10^{10}\,\dfrac{\text{gal}}{\text{yr}}[/tex]
