A project can be completed in 20 days by A alone, and 24 days by B alone. Now A and B cooperate, and B rests for a few days in the middle. In this way, it will be completed in 15 days. Find the number of days B rests.
一项工程，甲独做要20天完成，乙独做要24天完成，现在由甲乙合作，中间乙休息几天，这样共用15天完成，求乙休息的天数．
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</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 / 24</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">(1/4) / (1/24)</gadget>
<output>6</output>

<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<result>9</result>