Oh this is very easy!!
Answer: Based on the graph, the car requires 2.5 gallons of fuel to travel 100 miles.
Look at the picture below for help. The blue dot represents were the two points meet. We are trying to figure out how many gallons are for the 100 miles, 100 miles would be on the side that says distance. That is were you start. Find point 100 on the "distance" side and just draw a straight line, stopping when you hit the black line on the graph. Once done, draw another line going straight down from the black line, and you get your answer of 2.5
Answer: 2.5
Hope it helps!