A pool is filled in 6 minutes by opening pipe A alone, and emptied in 9 minutes by opening pipe B alone. How many minutes will it take to fill the pool if both pipes are open? How many.
一个水池，单开甲管6分钟注满，单开乙管9分钟排空．如果两管齐开，几分钟注满水池？多少．
<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>