There are 3 water pipes on a pool. Pipe A is the water inlet pipe, and pipe B is the water outlet pipe, which can drain the full pool of water in 20 minutes, and pipe C is also the water outlet pipe, and can drain the full pool of water in 30 minutes. Now open pipe A first. When the water in the pool just overflows, open pipe B and pipe C. It takes 18 minutes to drain the water. When pipe A is opened and filled with water, pipe B is opened again, and pipe C is not opened. How long does it take to drain the water? over?
一个池上装有3根水管．甲管为进水管，乙管为出水管，20分钟可将满池水放完，丙管也是出水管，30分钟可将满池水放完．现在先打开甲管，当水池水刚溢出时，打开乙，丙两管用了18分钟放完，当打开甲管注满水是，再打开乙管，而不开丙管，多少分钟将水放完？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / (1/12)</gadget>
<output>12</output>

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

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<gadget id="calculator">6 / 18</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(1/12) * (1/3)</gadget>
<output>1/36 = around 0.027778</output>

<gadget id="calculator">(1/20) - (1/36)</gadget>
<output>1/45 = around 0.022222</output>

<gadget id="calculator">1 / (1/45)</gadget>
<output>45</output>

<result>45</result>