Mary is interested in driving a new Honda Civic. It is worth $19,000 new and has a residual value of $13,000 after 3 years. She could take out a loan to buy the car. Her monthly loan payments would be $565.19. Alternatively, she could lease the car for $227.23 per month. Her friend tells her shell save $833.44 by buying the car. Explain mathematically what her friend means.