To process a batch of parts, A alone can complete it in 20 days, and B alone can complete (1/30) of this batch of parts every day. Now the two are working together to complete this batch of parts. A rested for 2.5 days, and B also rested for a few days. It took 15 days to complete all the parts. How many days did B rest?
加工一批零件，甲独做20天完成，乙独做每天完成这批零件的(1/30)．现在两人合作完成这批零件，甲中途休息了2.5天，乙也休息了若干天，这样用了15天才全部完成，求乙休息了几天？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">15 - 2.5</gadget>
<output>12.5</output>

<gadget id="calculator">(1/20) * 12.5</gadget>
<output>0.625</output>

<gadget id="calculator">1 - 0.625</gadget>
<output>0.375</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">0.375 / (1/30)</gadget>
<output>11.25</output>

<gadget id="calculator">15 - 11.25</gadget>
<output>3.75</output>

<result>3.75</result>