A pool has three water pipes A, B, and C. A, B are water inlet pipes, and C is a drain pipe. Opening pipe A alone can fill the pool in 100 minutes, and opening pipe B alone can fill the pool in 60 minutes. Opening alone Pipe C can drain the whole pool of water in 80 minutes. If both pipes A and C are opened at the same time, how long will it take to drain the whole pool of water?
一个水池有甲、乙、丙三个水管，甲、乙是进水管，丙是排水管，单独开放甲管100分钟可将水池注满，单独开放乙管60分钟可将水池注满，单独开放丙管80分钟可将全池水放完．若同时开放甲、丙两管，多少分钟可把全池水放完？
<gadget id="calculator">1 / 80</gadget>
<output>1/80 = around 0.0125</output>

<gadget id="calculator">1 / 100</gadget>
<output>1/100 = around 0.01</output>

<gadget id="calculator">(1/80) - (1/100)</gadget>
<output>1/400 = around 0.0025</output>

<gadget id="calculator">1 / (1/400)</gadget>
<output>400</output>

<result>400</result>