The graph shows the amount of fuel remaining in a car's tank, in gallons, as a function of the distance traveled, in miles. What is the car's fuel efficiency, in miles per gallon?

[Graph Description: A line segment starts at (0, 15) and goes down to (300, 0). The x-axis is 'Distance Traveled (miles)' from 0 to 350, with major ticks every 50 miles. The y-axis is 'Fuel Remaining (gallons)' from 0 to 20, with major ticks every 5 gallons. The line passes through (100, 10), (200, 5).]