A printer has a contract to print 100,000 posters for a political candidate. He can run the posters using any number of plates from 1 to 40 on his press. If he uses x plates, they will produce x copies of the poster with each impression of the press. The metal plates cost $15 to prepare and it costs $150 per hour to run the press. If the press can make 10,000 impressions per hour, how many plates should the printer make in order to minimize the cost of printing the 100,000 posters