The owner of a luxury motor yacht that sails among the 4000 Greek islands charges $600/person per day if exactly 20 people sign up for the cruise. However, if more than 20 people sign up for the cruise
(up to a maximum of 90), the fare for all the passengers is reduced by $4/person for each additional
passenger. Assume that at least 20 people sign up for the cruise, and let x denote the number of
passengers about 20. Hint: R(x) = (number of people above 20) x (price per person above 20)
a.) Find a function R giving the revenue per day realized from the charter