In a pool, groundwater infiltrates into the pool from the four walls, and the amount of water infiltrated per hour is fixed. Open pipe A, the full pool can be emptied in 8 hours, open pipe C, and the full pool can be emptied in 12 hours. If pipes A and B are opened, the water can be emptied in 4 hours. How many hours can the water be emptied by opening pipes B and C?
一个水池，地下水从四壁渗入池中，每小时渗入的水量是固定的．打开A管，8小时可以将满水池排空，打开C管，12小时可以将满水池排空．如果打开A、B管，4小时可将水排空，问，打开B、C管可以几小时将水排空？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(1/12) - (1/8)</gadget>
<output>-1/24 = around -0.041667</output>

<gadget id="calculator">(1/4) + (-1/24)</gadget>
<output>5/24 = around 0.208333</output>

<gadget id="calculator">1 / (5/24)</gadget>
<output>24/5 = around 4.8</output>

<result>24/5 = around 4.8</result>