high rise apartment complex has 550 apartment units when rent 1085 month all 550 units are

A high rise apartment complex has 550 apartment units. When the rent is 1085 per month all 550 units are occupied. Assume that for each 90 increase in rent 15 units becomes vacant. Assume that the number of occupied units is a linear function of the rent. Each occupied unit requires an average of 125 per month for service, repairs, and insureance. The rent charged should maximize the profits.