A job is 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 needs 6 hours to complete. How many hours does B need to do this work alone?
一件工作，甲、乙合作要4小时完成，乙、丙合作要5小时完成．现在先由甲、丙合作2小时后，余下的乙还需6小时完成，乙单独做这件工作要几小时？
<gadget id="calculator">6 - 2 - 2</gadget>
<output>2</output>

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

<gadget id="calculator">2 / 5</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">2 / (1/10)</gadget>
<output>20</output>

<result>20</result>