Burning one gallon of gasoline in a car releases approximately 20 pounds of CO₂ into the atmosphere. One person drives 60,000 miles in a car that averages 30 miles per gallon (mpg), while another person drives 60,000 miles in a car that averages 20 mpg. Over the course of the 60,000 miles, how many fewer pounds of CO₂ are released by the 30 mpg car than by the 20 mpg car?