To process a batch of parts, A alone can complete it in 6 hours, and B alone can complete it in 9 hours. How many hours can A and B work together to complete it?
加工一批零件，甲单独做6小时完成，乙单独做9小时完成，甲、乙合做多少小时可以完成．
<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>