There are two drainage pipes A, B, and one water inlet pipe C. If both pipes A and B are opened at the same time, the water in the pool can be emptied in 15 hours. If the pipes B and C are opened at the same time, the water in the pool can be emptied in 30 hours. If If pipe C is opened alone, 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 empty the full pool?
有两个排水管A、B，一个进水管C，若同时开A、B两管，15小时可将满池水排空，若同时开B、C两管30小时可将满池水排空，若单独开C管，60小时可将空池注满，若同时打开A、B、C三水管，要排空满池的水需要多少小时？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

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

<gadget id="calculator">(1/15) - (1/60)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / (1/20)</gadget>
<output>20</output>

<result>20</result>