The business manager of a 90 unit apartment building is trying to determine the rent to be charged. From past experience with similar buildings, when rent is set at $400, all the units are full. For every $20 increase in rent, one additional unit remains vacant. What rent should be charged for maximum total revenue? What is that maximum total revenue? Find the maximum revenue (or income)of the apartment building. What is the rent coincides with this maximum revenue? What is the outcome if the rent hikes of $20 results in 2 additional vacancies instead of 1?