There is a pool, one pipe A is opened for 5 minutes to fill the empty pool, pipe B is opened for 10 minutes to fill the empty pool, and pipe C is opened for 15 minutes to drain the full pool. Now the three pipes are open for 2 minutes and the second pipe is closed. How many minutes will it take to fill the pool?
有一水池，单开甲管5分钟注满空池，单开乙管10分钟注满空池，单开丙管15分钟可将满池水放尽．现在三管齐开2分钟后关闭乙管，问还要几分钟可注满水池？
<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>