A cruise company can either have two types of trips, a large cruise ship or a small cruise ship. A large cruise ship trip can carry 2000 customers and produces 20 units of pollution. A small cruise ship trip can carry 800 customers and produces 15 units of pollution. There can be at most 7 large cruise ship trips and at least 40% of the total trips must be made by small cruise ships. If the cruise company aims to transport at least 20000 customers, how many of each size of cruise ships should the company use to minimize the total amount of pollution produced?