job alone takes 20 days to complete, and B alone takes 28 days to complete. A and B cooperate. A rested for 3 days in the middle, and B also rested for a few days, so it took 18 days from the beginning to the end to complete. So how many days did B rest in the middle?
一件工作，甲单独做要20天完成，乙单独做要28天完成．甲乙两人合作．中间甲休息了3天，乙也休息了几天，所以从开始到结束共用了18天完成．那么乙中间休息了几天？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">18 - 3</gadget>
<output>15</output>

<gadget id="calculator">(1/20) * 15</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 28</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">(1/4) / (1/28)</gadget>
<output>7</output>

<gadget id="calculator">18 - 7</gadget>
<output>11</output>

<result>11</result>