A pool has inlet and outlet pipes. Open the water inlet pipe alone, and the empty pool can be filled with water in 4 hours; open the water outlet pipe alone, and the full pool can be drained in 6 hours. How long will it take to fill an empty pool with water if both the inlet and outlet pipes are turned on at the same time?
一个水池装有进水管和出水管．单开进水管，4小时可以将空池注满水；单开出水管，6小时可以将满池水放完．如果同时打开进水管和出水管，需要多少时间才能将空池注满水？
<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>