A project alone takes 30 days, and B alone takes 40 days. A and B cooperate to complete the project. During this process, A rests for 3 days, and B also rests for a few days. Finally, the project is completed in 21 days. , then how many days did B rest.
一项工程，甲独做要30天，乙独做要40天，甲乙合作来完成这项工程，在这个过程中甲休息了3天，乙也休息了几天，最后在21天完成了工程，那么乙休息了多少天．
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<gadget id="calculator">(1/30) * 18</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(2/5) / (1/40)</gadget>
<output>16</output>

<gadget id="calculator">21 - 16</gadget>
<output>5</output>

<result>5</result>