A pool has two inlet pipes and one outlet pipe. Open pipe A for 12 hours to fill the empty pool, open pipe B for 15 hours to drain the full pool, and open pipe C for 20 hours to fill the empty pool. When the three pipes are opened at the same time, how many hours can the empty tank be filled with water?
一个水池有两个进水管，一个出水管．开甲管12小时可把空池注满，开乙管15小时可把满池水放完，开丙管20小时可把空池注满．三管同时开，多少小时可把空池注满水？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/20) - (1/15)</gadget>
<output>-1/60 = around -0.016667</output>

<gadget id="calculator">(1/12) + (-1/60)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / (1/15)</gadget>
<output>15</output>

<result>15</result>