A task can be completed by A alone in 10 days, and by B alone in 20 days. Now A works for 4 days first, and the remaining cooperation between A and B, how many days will it take to complete?
一项任务，甲单独做10天可以完成，乙单独做20天可以完成．现由甲先做4天，剩下的甲、乙合作，还需要多少天才能完成？
<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>

<result>4</result>