project can be completed in 10 days, B in 15 days, and C in 20 days. After A and B cooperate for 3 days, C will do the rest alone. How many days will it take to complete?
一项工程，甲独做要10天完成，乙独做要15天完成，丙独做要20天完成．甲、乙合作3天后，余下的由丙独做，还要多少天完成？
<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">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) * 3</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/2)</gadget>
<output>1/2 = around 0.5</output>

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

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

<result>10</result>