In Example 7, find the relationship between n and x in the two methods used to solve the problem.
Example 7
BBC Rentals Inc. owns a large apartment complex containing 400 identical apartments. If the monthly rent for each apartment is $800 all the apartments are rented. However, for each $60 monthly increase in rent 3 apartments become vacant. Find an expression which may be used to represent their monthly income and determine the monthly rent to be charged to maximize their income