Frodo and Sam are photographers. They take pictures of seniors for high school yearbooks. They charge a standard fee for taking the pictures. There's also a charge for each set of prints ordered. So the number of sets of prints determines the total cost.

Let S stand for the sets of prints ordered. Write the rule that describes a student's total cost.

Sets of prints Total cost
0 $15
1 $19
2 $23
3 $27