A pool is equipped with a water inlet pipe and an outlet pipe. A single water inlet pipe can fill a pool of water in 6 hours; a single water outlet pipe can fill a pool of water in 9 hours. How long does it take to fill half of the pool with two pipes open at the same time?
一个水池装有进水管和出水管各一根，单根开进水管，6小时可以放满一池水；单独开出水管，9小时可以放完一池水．两管同时开，放满半池水需要多少时间？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<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>1/18 = around 0.055556</output>

<gadget id="calculator">(1/2) / (1/18)</gadget>
<output>9</output>

<result>9</result>