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 a maximum of 90) for the cruise, then the fare is reduced by $5/passenger for each passenger above 20. (That is, with 30 passengers, the fare for all passengers would be 600 - 5*10 = $550/passenger.) Please advise how to write equation that will calculate the total revenue R(x), where x is the number of passengers over 20. Also, what is the revenue for a day when there are 60 passengers?