certain project, A, B, and C alone need 10 days, 15 days, and 20 days to complete. Now three people are working together, A will take a break for 1 day, B will rest for 3 days, and C will work until the completion. How many days did it take to complete the task?
某工程，甲、乙、丙单独做各需10天、15天、20天才完成．现三人合做，中途甲先休息1天，乙再休息3天，而丙一直工作到完成为止．完成任务时一共用了多少天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

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

<gadget id="calculator">(1/15) * 3</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 + (1/10) + (1/5)</gadget>
<output>13/10 = around 1.3</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">(13/10) / (13/60)</gadget>
<output>6</output>

<result>6</result>