A manuscript takes 5 hours to copy alone, and B takes 4 hours to copy alone. How many hours will it take for them to copy together?
一份稿件，甲单独抄完要5小时，乙单独抄完要4小时，他们一起抄完要多少小时
<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>