Charter revenue the owner of a luxury motor yacht that sails among the 4000 greek islands charges $600/person/ day if exactly 20 people sign up for the cruise. However, if more than 20 people sign up (up to the maximum capacity of 90) for the cruise, then each fare is reduced by $4 for each additional passenger. Assume at least 20 people sign up for the cruise and let x denote the number of passengers above 20.
A. Find a function r giving the revenue/day realized from the charter.
B. What is the revenue/day if 60 people sign up for the cruise?
C. What is the revenue/day if 80 people sign up for the cruise?