Jason is going to play golf this afternoon and has to choose between two different golf courses.

The greens fees to play each are both $18, the cart fees at each course is $9.

The Eagle's Ridge course is located 20 miles further away than the Crescent course.

Therefore it will cost approximately an additional $4 in gasoline to play the Eagle's Ridge course.

What are the total relevant costs associated with playing golf at Eagle's Ridge?

the answer is $4, please explain. It asks for total relevant cost, why don't you add all the relevant cost?