Using a computer to enter a manuscript, A can complete it in 10 days alone, and B can complete it in 20 days alone. Now A and B are working together. Because B fell ill on the way and rested for several days, it took 8 days to complete the task. So how many days did B rest in the middle?
用计算机录入一份书稿，甲单独做10天可以完成，乙单独做20天可以完成，现在由甲、乙两人合作，由于乙途中生病休息了若干天，结果一共用了8天才完成任务，那么乙中途休息了多少天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 8</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/5) / (1/20)</gadget>
<output>4</output>

<gadget id="calculator">8 - 4</gadget>
<output>4</output>

<result>4</result>