18. A weeklong summer camp is held in June for children in Grades 3-6. Parents and guardians who enrolled their children for camp by May 15 received a 20% discount off the regular enrollment fee for each child enrolled. For each grade, the table below gives the number of children enrolled by May 15 as well as the regular enrollment fee per child. The grade of any child is that child’s grade in school as of May 15.

Which of the following equations gives a true relationship between the regular enrollment fee, f, and the grade, g, of any child enrolled in the camp?