In addition to gas needed for personal transportation, there is also the cost of gas for home use, and any gas used by municipalities or other publicly-funded services. That said, I do believe the desired response would include guestimating and showing your reasoning for each part of the answer.

They're trying to test your estimation & reasoning skills. Even if you're wrong, estimate the population of Dallas, the proportion of regular drivers, the average number of miles driven per day * 365, the average mpg (which would be quite high in Dallas given the number of big cars there) and the cost per gallon. Before the interview, practice estimation skills in general in various situations.

Me: More than what they used to.

about $3.50 per gallon

They're trying to test your estimation & reasoning skills. Even if you're wrong, estimate the population of Dallas, the proportion of regular drivers, the average number of miles driven per day * 365, the average mpg (which would be quite high in Dallas given the number of big cars there) and the cost per gallon. Before the interview, practice estimation skills in general in various situations.

BTW, the question says "gas", not "gasoline".

Depends. Was this vehicle gasoline or airplane gasoline? I'm assuming because I'm interviewing for American Airlines, that you're not referring to the assumed car gasoline. Since 27 Million people flew from DFW in 2008, and the average domestic airfare in 2008 was $350, calculating that gas accounts for about 35% of airline ticket price, then the average money residents of DFW spent on gasoline in 2008 was $330,750,000. When do I start?

More than they had to. I'm sure there are plenty of ways to reduce consumption and therefore cost without sacrificing any functionality. The resulting environmental benefits notwithstanding.

Enough to pay for the gasoline they needed.

Less than the people in Chicago.

Too much?

It depends on many factors. For E.G. 1. How many miles each user is driving. 2. What kind of vehicle users drive around? 3. How expensive is gas in the area.

They payed what they thought it was worth to perform the tasks at hand.