cistern has one inlet pipe and three outlet pipes that discharge water at the same rate. Opening only one inlet pipe can fill the empty pool in 20 minutes, and opening one outlet pipe alone can fill the pool with water in 45 minutes. The water in the existing (2/3) pool, if the four pipes are opened together, after how many minutes there will be 2/5 of the water in the pool.
一个蓄水池装了一根进水管和三根放水速度一样的出水管．单开一根进水管20分钟可注满空池，单开一根出水管45分钟可放完满池的水．现有(2/3)池的水，如果四管齐开，多少分钟后池水还剩2/5．
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/3) - (2/5)</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">1 / 45</gadget>
<output>1/45 = around 0.022222</output>

<gadget id="calculator">(1/45) * 3</gadget>
<output>1/15 = around 0.066667</output>

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

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

<gadget id="calculator">(4/15) / (1/60)</gadget>
<output>16</output>

<result>16</result>