"Do you understand how we got that figure? First you find out
the total number of gallons of gasoline your family runs through in
a year. If necessary, you can divide the total number of miles driven
by your car's miles-per-gallon average...
"Or you could take the total amount your family spends on gasoline
per year, and divide it by the average price of a gallon of gas. This
will also tell you how many gallons of gas your family consumes in
a year.
"Once you know how much gas your family uses, then you divide
the total number of gallons by the number of people in your family,
to find out what your share is. In Jeff's case, it's 2177 gallons
divided by 3 = 726 gallons for him per year."