For a job, A and B worked together for 4 hours on the first day, and completed all the work (2/5); on the second day, B worked alone for 7 hours, and the rest of the work was not completed (11/30) ． How many hours will it take for A to do this job alone?
一项工作，第一天甲、乙两人合做4小时，完成全部工作的(2/5)；第二天乙又独做了7小时，还剩全部工作的没完成(11/30)．这件工作由甲一人独做需要多少小时？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) / 4</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">11 / 30</gadget>
<output>11/30 = around 0.366667</output>

<gadget id="calculator">1 - (11/30) - (2/5)</gadget>
<output>7/30 = around 0.233333</output>

<gadget id="calculator">(7/30) / 7</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/10) - (1/30)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / (1/15)</gadget>
<output>15</output>

<result>15</result>