A project alone takes 5 days, and B alone takes 4 days. If A and B work together on this project, how many days can they complete the task?
一项工程，甲独做要5天，乙独做要4天，甲、乙合做这项工程，多少天可以完成任务．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">1 / (9/20)</gadget>
<output>20/9 = around 2.222222</output>

<result>20/9 = around 2.222222</result>