A project can be completed in 4 hours by the cooperation of A and B, and 5 hours by the cooperation of B and C. Now after A and C cooperate for 2 hours, the remaining B will complete in 6 hours. How many hours will it take for B to complete it alone?
一项工程，甲乙合作4小时完成，乙丙合作5小时完成，现在甲丙合作2小时后，余下的乙还要6小时完成，如果由乙单独做要几个小时完成？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) * 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) * 2</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">6 - 2 - 2</gadget>
<output>2</output>

<gadget id="calculator">(1/10) / 2</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / (1/20)</gadget>
<output>20</output>

<result>20</result>