A concert promoter charges $65 per ticket for a concert. The promoter's cost will be $5500 plus $4 per ticket. If r = revenue and n = number of tickets sold, then r(n) shows how revenue (amount of money received from ticket sales) depends on the number of tickets sold.

r(n) = 65n

If c = cost and n = number of tickets sold, then c(n) shows how cost (expenses of the concert promoter) depends on the number of tickets sold.

c(n) = 4n + 5500