There is a pool, equipped with two water injection pipes A and B, below which is installed pipe C to discharge water. When the pool is empty, it can be filled in 5 minutes by opening tube A alone; it can be filled in 10 minutes by opening tube B alone. After the pool is filled with water, just open the pipe C for 15 minutes to drain the water. If the pool is empty, pipes A, B, and C are opened together, and pipe B is closed after 2 minutes, how many minutes will it take to fill the pool.
有一水池，装有甲、乙两个注水管，下面装有丙管放水．池空时，单开甲管5分钟可注满；单开乙管10分钟可注满．水池装满水后，单开丙管15分钟可将水放完．如果在池空时，将甲、乙、丙三管齐开，2分钟后关闭乙管，还要多少分钟可以注满水池．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) - (1/15)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/5) + (1/30)</gadget>
<output>7/30 = around 0.233333</output>

<gadget id="calculator">(7/30) * 2</gadget>
<output>7/15 = around 0.466667</output>

<gadget id="calculator">1 - (7/15)</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">(1/5) - (1/15)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">(8/15) / (2/15)</gadget>
<output>4</output>

<result>4</result>