There is a water inlet pipe in a pool. Now, two holes of the same size are drilled side by side at the middle height of one side of the pool. If only the water inlet pipe is opened, the pool can be filled in 40 minutes. If one hole and water inlet pipe are opened, the pool can be filled in 50 minutes. If two holes and water inlet pipes are opened, how many minutes can the pool be filled?
一个水池有一个进水管，现在在水池的一个侧面的正中间高度并排打2个相同大小的孔，如果只打开进水管，40分钟能够灌满水池．如果打开一个孔和进水管，那50分钟可以灌满水池，如果打开两个孔和进水管，多少分钟能够灌满水池？
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

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

<gadget id="calculator">50 - 20</gadget>
<output>30</output>

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

<gadget id="calculator">(1/20) - (1/30)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">2 * (1/60) * (1/2)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/40) - (1/60)</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">(1/2) / (1/120)</gadget>
<output>60</output>

<gadget id="calculator">20 + 60</gadget>
<output>80</output>

<result>80</result>