Part I (6 points each, total 30 points)
1. Xiao Gang goes to buy milk and finds that the milk is on special offer today, with each bag costing 2 yuan and 50 cents, and buy two get one free. Xiao Gang has 30 yuan. What is the maximum number of bags of milk he can buy? $\qquad$
【Analysis】 $30 \div 5 \times 3=18$ (bags)