project alone can be completed in 10 hours, B in 15 hours, and C in 20 hours. Now the three are working together, and A stops work for a few hours because of something, and the work is completed in 6 hours. How many hours does A stop working?
一项工程，甲单独完成需要10小时，乙单独完成需要15小时，丙单独完成需要20小时．现三人合作，中途甲因有事停工几小时，结果6小时才将工作完成．问甲停工几小时？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) + (1/20)</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">(3/10) / (1/10)</gadget>
<output>3</output>

<gadget id="calculator">6 - 3</gadget>
<output>3</output>

<result>3</result>