There is a project that team A can complete in 20 days alone, and team B can complete it in 30 days alone. Now the two teams cooperate to do this project. During the cooperation, team A rests for 4 days, and team B rests for several days. A total of 15 days to complete the work, how many days did Team B rest?
有项工程，甲队单独做20天可以完成，乙队单独做30天可以完成，现在由两队合作来做这项工程，合作中，甲队休息了4天，乙队休息了若干天，共15天完工，则乙队休息了多少天．
<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">(1/12) * 15</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">(1/20) * 4</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(5/4) - 1 - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) / (1/30)</gadget>
<output>3/2 = around 1.5</output>

<result>3/2 = around 1.5</result>