A project alone takes 10 days to complete, B alone takes 15 days to complete, and C alone takes 20 days to complete. After three days of cooperation, A has other tasks and quits, leaving B and C to continue working until completion. How many days will it take to complete this project.
一项工程，甲单独完成需10天，乙单独完成需15天，丙单独完成需20天，三人合作3天后，甲有其它任务而退出，剩下乙、丙继续工作直至完工．完成这项工程共用多少天．
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<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">(1/10) + (1/15) + (1/20)</gadget>
<output>13/60 = around 0.216667</output>

<gadget id="calculator">(13/60) * 3</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">1 - (13/20)</gadget>
<output>7/20 = around 0.35</output>

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

<gadget id="calculator">(7/20) / (7/60)</gadget>
<output>3</output>

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

<result>6</result>