R - total revenue
Revenue without changing the rent:
R = $496 · 90 = $44,640
x - number of dollars increased
R = ( 90 - x/8 ) ( 496 + x )
R = - x²/8 + 28 x + 44,640
R` = - x/4 + 28
- x/4 + 28 = 0
x/4 = 28
x = 28 · 4 = $112
Maximum revenue: R = 112 · 76 = $46,208
The rent should be:
$112 + $496 = $608 per month