There are two water pipes in a pool. The empty pool can be filled in 6 hours by only opening the A water pipe, and the full water can be filled by only opening the B water pipe in 9 hours. If both pipes are opened together, how many hours can the empty pool be filled?
一个水池有两水管，单开甲水管6小时可以把空水池放满，单开乙水管9小时可以把满水池放完，两管齐开，多少小时可以把空池注满？
<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 / (1/18)</gadget>
<output>18</output>

<result>18</result>