There is a working group. When each worker is working in their respective jobs, a batch of parts can be produced in 7 hours. If the positions of workers A and B are exchanged and the others remain unchanged, the batch of parts can be completed 1 hour earlier. , If the positions of workers C and D are exchanged, and the others remain unchanged, it can also be advanced 1 hour. If the positions of A and B, C and D are exchanged at the same time, and the others remain unchanged, how long will it take to complete this batch of parts? ．
有一个工作小组，当每个工人在各自的工作岗位上工作时，7小时可生产一批零件，如果交换工人甲、乙的岗位，其他人不变，那么可提前1小时，完成这批零件，如果交换工人丙、丁的岗位，其他人不变，也可提前1小时，问如果同时交换甲与乙、丙与丁的岗位，其他人不变，那么完成这批零件需多长的时间．
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) - (1/7)</gadget>
<output>1/42 = around 0.02381</output>

<gadget id="calculator">(1/42) * 2</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">(1/7) + (1/21)</gadget>
<output>4/21 = around 0.190476</output>

<gadget id="calculator">1 / (4/21)</gadget>
<output>21/4 = around 5.25</output>

<result>21/4 = around 5.25</result>