The manager of a large apartment complex knows from experience that 90 units will be occupied if the rent is 400 dollars per month. A market survey suggests that, on the average, one additional unit will remain vacant for each 10 dollar increase in rent. Similarly, one additional unit will be occupied for each 10 dollar decrease in rent. What rent should the manager charge to maximize revenue? All I know is that 90 units will give me $36000 What do I have to do after? I am so stuck on this problem I have no idea what to do.