A pool has two drain pipes, A and B, and one inlet pipe, C. If both pipes A and C are opened at the same time, the full pool can be emptied in 20 hours; if both pipes B and C are opened at the same time, the full pool can be emptied in 30 hours; if only pipe C is opened, the empty pool can be filled in 60 hours; If pipes A, B, and C are opened at the same time, how many hours will it take to drain the water in the pool?
一个水池有甲乙两个排水管，一个进水管丙．若同时开放甲丙两管，20小时可将满池水排空；若同时开放乙丙两水管，30小时可将满池水排空；若单开丙管，60小时可将空池注满；若同时打开甲乙丙三水管，要排空水池中的水需多少小时．
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

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

<gadget id="calculator">(1/60) - (1/60)</gadget>
<output>0</output>

<gadget id="calculator">(1/20) + (1/60) + (1/30) + 0</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / (1/10)</gadget>
<output>10</output>

<result>10</result>