In a 100-unit apartment building when the price charged per apartment rental is (820 + 20x) dollars, the number of apartments rented is 100 - x and the total revenue for the building is
R(x) = (820 + 20x)(100 - x),
where x is the number of $20 rent increases (and the resulting number of unrented apartments). Find the marginal revenue (in dollars) when x = 10.
$_______