A project alone takes 4 hours for A and 5 hours for B, how much can A and B cooperate to complete the project per hour.
一项工程，单独做甲要4小时，乙要5小时，甲乙合作每小时完成这项工程的多少．
<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>

<result>9/20 = around 0.45</result>