A small chocolate shop makes milk chocolate and dark chocolate bars. Milk chocolate bars require 4 units of cocoa and 7 units of milk. Dark chocolate bars require 6 units of cocoa and 3 units of milk. The shop has 2000 units of cocoa and 1750 units of milk available. In addition since milk chocolate sells better, at least 2 times as many milk chocolate bars need to be made as dark chocolate bars.  If making a milk chocolate bar takes 15 minutes and making a dark chocolate bar takes 12 mins, how many of each should the make to minimize total production time?