A project can be completed by A in 10 days and by B in 20 days. If A works alone for 4 days first, and A and B jointly do the rest, how many days will it take to complete the project?
一项工程，甲独做需10天完成，乙独做需20天完成．如果先由甲独做4天，剩下的由甲乙合做，完成这项工程共需多少天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 4</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

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

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

<gadget id="calculator">(3/5) / (3/20)</gadget>
<output>4</output>

<gadget id="calculator">4 + 4</gadget>
<output>8</output>

<result>8</result>