Question

Assume there are 100 million passenger cars in the
United States and that the average fuel consumption is
20 mi/gal of gasoline. If the average distance traveled
by each car is 10 000 mi/yr, how much gasoline would
be saved per year if average fuel consumption could be
increased to 25 mi/gal?

Answer 1

1000000000 gal would be saved per year

Answer 2

All you need to to is calculate the difference between the two consumptions:
\[\frac{10000mi}{20mpg}*100,000,000=50,000,000,000g\]\[\frac{10000mi}{25mpg}*100,000,000=40,000,000,000g\]So 10 billion gallons of fuel would be saved if the mpg was increased to 25.