and B start to process a batch of parts at the same time. A alone will take 20 hours, and B alone will take 30 hours. Two people work together, A rests for 4 hours, and completes the task after 15 hours, so how many hours does B rest?
甲、乙二人同时开始加工一批零件，甲单独做要20小时，乙单独做要30小时．两人合做中甲休息了4小时，15小时后完成任务，那么乙休息了几小时？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">15 - 4</gadget>
<output>11</output>

<gadget id="calculator">(1/20) * 11</gadget>
<output>11/20 = around 0.55</output>

<gadget id="calculator">1 - (11/20)</gadget>
<output>9/20 = around 0.45</output>

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

<gadget id="calculator">(9/20) / (1/30)</gadget>
<output>27/2 = around 13.5</output>

<gadget id="calculator">15 - (27/2)</gadget>
<output>3/2 = around 1.5</output>

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