The manager of a 100-unit apartment complex knows from experience that all units will be occupied if the rent is $800 per month. a market survey suggests that, on average, one additional unit will remain vacant for each $10 increase in rent. what rent should the manager charge to maximize revenue?

Two farmers pay a total rent of $140 for a pasture. the first farmer placed $4 oxen in the pasture, and the second farmer placed $30 sheep in the pasture. the farmers split the rent based on the proportion of the pasture eaten by their own animals that month. how much should the first farmer pay, if 1 ox eats as much as 10 sheep in a month?