This is Giancoli Answers with Mr. Dychko. To figure out the total amount of gas burned by all drivers in the US, we'll have to figure out or estimate the number of drivers that there are and multiply that by the number of miles each driver drives per year and then multiply it by the average fuel efficiency of vehicles in the US. And it turns out that a lot of this information is available; the government keeps statistics on the number of drivers so this website—department of transportation, dot.gov— Federal Highway Administration, it has the number of licensed drivers in the US. and looking way down here, we have number of drivers is this third column here and in 2010, there were 210 million drivers; this is in millions as you can see from the axis label on the y-axis there. So that's 210 million drivers in the US in 2010. So that's where this 210 times 10 to the 6 comes from, 210 million drivers. We'll just say the average distance per year for each driver is 15000 miles per year; average fuel efficiency, I had to look that one up too and the government also has stats on that so over here in at the Environmental Protection Agency, they have fuel efficiency trends; in 2012, average fuel economy was 23.6 miles per gallon. So plug that number in and we have 210 times 10 to the 6 drivers times 15000 miles per driver and then 1 gallon for every 23.6 miles we have units on opposite sides of the fraction canceling here leaving us with gallons per year for all drivers combined. So we have 1.3 times 10 to the 11 gallons per year so that's 130 billion gallons per year is our estimate. And we can compare that with the reality because the government keeps really careful track of how much gas is taxed in the US. So this is highway statistics series again and net gallons taxed every year and looking at 2010, that's 170 million here but this is thousands of gallons so put another three zeros on the end of this; this is 170 billion gallons of gas taxed in the US in 2010. So that's a 170 billion compared to our 130 billion and we actually expect our estimate to be a little bit low because we have not accounted for commercial drivers like truckers who will go a lot farther than 15000 miles per year we haven't accounted for airplanes, you know, all the gas consumed by all those jets is not included here. So I guess that would make up for the difference perhaps of this 0.4 billion gallons per year that we didn't capture in our estimates here.