If A takes 4 hours to complete a job alone, and B takes 5 hours to complete it alone, how many hours will it take for the two to complete the job together?
一件工作甲单独完成需要4小时，乙单独完成需要5小时，两人合作需要几小时完成这份工作？
<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">1 / (9/20)</gadget>
<output>20/9 = around 2.222222</output>

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