A project alone needs 20 days to complete, and B completes (1/30) of this project every day. Now they are working together, and A rests for 2.5 days, and B also rests for several days, which is delayed by 3 days compared to the original plan. The work is completed in days, how many days does B rest?
一工程，甲单独做需要20天完工，乙每天完成这件工程的(1/30)，现在两人合作，中途甲休息2.5天，乙也休息了若干天，这样比原计划拖迟了3天完工，求乙休息的天数？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / (1/12)</gadget>
<output>12</output>

<gadget id="calculator">3 - 2.5</gadget>
<output>0.5</output>

<gadget id="calculator">12 + 0.5</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">0.375 / (1/30)</gadget>
<output>11.25</output>

<gadget id="calculator">3 - 11.25</gadget>
<output>-8.25</output>

<gadget id="calculator">12 + (-8.25)</gadget>
<output>3.75</output>

<result>3.75</result>