The equation to this problem is:
$30 + $0.35 . x = $40.85
where x is the amount of minutes that exceeds the 500-minute free limit.
$30 + $0.35 . x = $40.85
$0.35 . x= $40.85 - $30
x = $10.85 / $0.35
x = 31
The total amount of minutes used is 31 (charged) + 500 (free) = 531.
Response: 531 minutes.
:-)