A real estate office handles an apartment building that has 50 units. When the rent is $540

Question:

A real estate office handles an apartment building that has 50 units. When the rent is $540 per month, all units are occupied. However, for each $30 increase in rent, one unit becomes vacant. Each occupied unit requires an average of $18 per month for service and repairs. What rent should they charge to obtain the maximum profit?