A policeman cruises (on average) approximately 10 min before stopping a car for speeding. Ninety percent of the cars stopped are given speeding tickets with an

$80 fine. It takes the policeman an average of 5min to write such a ticket. The other 10% of the stops are for more serious offenses, leading to an average fine of $300.

These more serious charges take an average of 30 min to process. In the long run, at what rate (in dollars per minute) does he assign fines.