It takes 4 hours for Xiao Wang to enter alone, and 5 hours for Xiao Zhang to enter. How many hours does it take for Xiao Wang and Xiao Zhang to enter a document together?
小王单独录入要4小时，小张录入要5小时．录入一份文件，小王和小张两人一起录入需要多少小时才能完成？
<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>