imc

Question

Elmer's new car gives 50% percent better fuel efficiency, measured in kilometers per liter, than his old car. However, his new car uses diesel fuel, which is 20% more expensive per liter than the gasoline his old car used. By what percent will Elmer save money if he uses his new car instead of his old car for a long trip?

Accepted Answer

Suppose that his old car runs at x km per liter. Then his new car runs at 32x km per liter, or x km per 23 of a liter. Let the cost of the old car's fuel be c, so the trip in the old car takes xc dollars, while the trip in the new car takes 2365xc = 45xc. He saves 15xcxc = 20%.