The revenue for a sale of x books at $80 each is given by R(x) = 80x. The commission for a revenue of x dollars is given
by C(x) = 0.30x. Express the commission as a function of
the number of books sold
since C is a function of revenue,
C(R(x)) = 0.30R(x) = 0.30(80x) = 24x
the "x" in this equation is the number of books.