It takes 4 days to complete a task alone, and 5 days to complete it alone. If two people work together in 2 days, how much of this task can be completed.
做一件工作，甲独做要4天完成，乙独做要5天完成，如果两人一起做2天，就能完成这件工作的多少．
<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">2 * (9/20)</gadget>
<output>9/10 = around 0.9</output>

<result>9/10 = around 0.9</result>