how do i write this in equation of this situation

Question

The cost C of a telegram of 18 words is the cost F of the first 10 words plus the cost A of each additional word.

Answer 1

c=f+8a

Answer 2

is it
18=F(10+A)

Answer 3

no, it wouldn't be that

Answer 4

For n words:

C(n)=F if n<=10
C(n)=F+A(n-10) if n>10