A pool has two inlet pipes, A and B. When pipe A is opened alone, the pool can be filled with water in 6 hours, and pipe B can be filled with water in 9 hours. How many hours can a pool of water be filled with two pipes open?
一个水池有甲乙两个进水管，独开甲管6小时可以注满一池水，独开乙管9小时可以注满一池水．两管齐开，多少小时可以注满一池水？
<gadget id="calculator">6 / 1</gadget>
<output>6</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

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