How many gallons of gasoline are used in the united states in one day? assume that there are two cars for every three people, that each car is driven an average of 10,000 miles per year, and that the average car gets 20 miles per gallon?

Considering the population of U.S. being 323,000,000 we will have the statement:
3 people .................................... 2 cars 
323,000,000 people .................. x cars
=>  cars.
Next we will found out the miles driven in one year by all the cars:
215,333,333 * 10000 = 2,153,333,330,000 miles/ year.
2,153,333,330,000 : 365 = 5,899,543,378 miles/ day.
5,899,543,378  : 20 = 294,977,168 gallons.