Three workers A, B, and C are going to turn out 7 parts on 3 lathes with the same efficiency. The time required to process each part is 6, 7, 7, 8, 8, 9, and 9 minutes respectively. 3 people start working at the same time, how long is the minimum time to finish all the parts?
甲、乙、丙3名工人准备在同样效率的3个车床上车出7个零件，加工各零件所需要的时间分别为6，7，7，8，8，9，9分钟．3人同时开始工作，问最少经过多少分钟可车完全部零件？
<result>20</result>