On the weekend, Xiao Ming went to McDonald's and found that the A-type set meal was 18 yuan each, and the B-type set meal was 21 yuan each. He brought 60 yuan and bought 3 sets. How many ways can he buy?
周末，小明去麦当劳，发现A类套餐每份18元，B类套餐每份21元，他带了60元，买3份，有几种买法？
<result>3</result>