A project can be completed in 6 days alone, and B can be completed in 9 days alone. How many days will A and B cooperate to complete the project?
一项工程甲独做6天完成，乙独做9天完成．甲乙合作多少天完成这项工程．
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/6) + (1/9)</gadget>
<output>5/18 = around 0.277778</output>

<gadget id="calculator">1 / (5/18)</gadget>
<output>18/5 = around 3.6</output>

<result>18/5 = around 3.6</result>