The manager of a large apartment complex knows from experience that 100 units will be occupied if the rent is 420 dollars per month. A market survey suggests that, on the average, one additional unit will remain vacant for each 6 dollar increase in rent. Similarly, one additional unit will be occupied for each 6 dollar decrease in rent. What rent should the manager charge to maximize revenue? Answer: $ [Recall: Revenue is the product of price per unit and number of units rented.]