A certain project can be completed by A alone for 40 days, and then by B in 28 days. Now the cooperation between A and B will be completed within 35 days. If A works alone for 30 days first, and then B continues, how many days will B have to work to complete?
某项工程先由甲单独做40天，再由乙做28天可以完成．现在甲、乙合作35天就完成了．如果先由甲单独做30天，再由乙接着做，乙还要工作多少天才能完成？
<gadget id="calculator">1 / 35</gadget>
<output>1/35 = around 0.028571</output>

<gadget id="calculator">(1/35) * 28</gadget>
<output>4/5 = around 0.8</output>

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

<gadget id="calculator">40 - 28</gadget>
<output>12</output>

<gadget id="calculator">(1/5) / 12</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/60) * 30</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/2)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/35) - (1/60)</gadget>
<output>1/84 = around 0.011905</output>

<gadget id="calculator">(1/2) / (1/84)</gadget>
<output>42</output>

<result>42</result>