and B work together for 6 hours to complete a job. Now, A will do 2 hours first, and B will do 4 hours, which just completes 50% of the job. How many hours will it take for B to complete the job alone?
甲、乙合做6小时可完成一项工作，现由甲先做2小时，再由乙做4小时，刚好完成这项工作的50%，若乙单独完成这项工作需要多少小时？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/6) * 2</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(1/2) - (1/3)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">(1/6) / 2</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/6) - (1/12)</gadget>
<output>1/12 = around 0.083333</output>

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

<result>12</result>