A project can be completed in 4 days by A alone, 5 days by B alone, and 2 days after the two teams work together, A will continue to complete the project due to some circumstances, how many days will B have to work to complete the task.
一项工程，甲独做4天完成，乙独做5天完成，两队合做2天后，甲因事由乙接着完成，乙还要做多少天才能完成任务．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) + (1/5)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(9/20) * 2</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) / (1/5)</gadget>
<output>1/2 = around 0.5</output>

<result>1/2 = around 0.5</result>