pool has two water pipes, one water inlet pipe and one drain pipe. If the water inlet pipe is opened alone, it can be filled with water in 4 hours. If the drain pipe is opened alone, the whole pool can be drained in 6 hours. Open, how many hours can be filled with water?
一个水池有两个水管，一个进水管，一个排水管，如果单独开进水管，4个小时就可以注满水，如果单独开排管，6小时就可以将整池水排完，现两管齐开，几小时才可注满水？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">(1/4) - (1/6)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / (1/12)</gadget>
<output>12</output>

<result>12</result>