To process a batch of parts, A, B, and C need 20 hours, 10 hours, and 15 hours respectively. Now this project is first cooperated by A and C for 6 hours, and then C is transferred away, and A and B cooperate. How many hours can B cooperate to complete the task?
加工一批零件，甲、乙、丙三人单独分别需要20小时、10小时、15小时，现在这项工程先由甲、丙合作6小时，接着丙调离，由甲、乙合作，甲、乙合作几小时才能完成任务？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/20) + (1/15)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(7/60) * 6</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 - (7/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/20) + (1/10)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/10) / (3/20)</gadget>
<output>2</output>

<result>2</result>