Three lathe workers A, B, and C are going to process seven parts on three lathes with the same efficiency. The processing time of each part is 4, 5, 6, 6, 8, 9, and 9 minutes respectively. The three workers start working at the same time ． Q: How long does it take at least to process seven parts?
甲、乙、丙三名车工准备在同样效率的3个车床上加工七个零件，各零件加工所需时间分别为4，5，6，6，8，9，9分钟，三人同时开始工作．问：加工完七个零件最少需多长时间？
<gadget id="calculator">8 + 5</gadget>
<output>13</output>

<gadget id="calculator">13 + 4</gadget>
<output>17</output>

<result>17</result>