A magic school sends letters to student either by carrier pigeons or owls. A carrier pigeon can carry two letters at a time and requires 3 treats for service. An owl can carry 5 letters at a time and requires 5 treats for service.  At most 40% of the birds can be owls. In addition, the school only has 1000 treats available and at least 20 carrier pigeons must be uses. How many of each bird can be used to maximize the total number of letters that can be sent.