A pool has three water pipes A, B, and C. A is the water inlet pipe, and B and C are the drain pipes. Opening pipe A alone can fill the pool in 5 minutes, and opening pipe B alone can drain the water in 20 minutes. Turning on the pipe C can drain the full pool of water in 8 minutes, how many minutes will it take to fill an empty pool by opening three water pipes at the same time?
一个水池有甲、乙、丙三个水管，甲是进水管，乙、丙是排水管，单开甲管5分钟可将水池注满，单开乙管20分钟可将满池水放完，单开丙管8分钟可将满池水放完，问同时打开三个水管，多少分钟才能将一个空水池注满？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(1/5) - (1/20) - (1/8)</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">1 / (1/40)</gadget>
<output>40</output>

<result>40</result>