A pool has two water inlet pipes A and B and one outlet pipe C, the pool can be filled with water in 20 hours by opening pipe A alone, the pool can be filled with water in 30 hours by opening pipe B alone, and the water can be filled by opening pipe C in 60 hours. The pool is full of water. Now that the three pipes are opened at the same time, how many hours can the pool be filled with water?
一个水池有甲、乙两个进水管和丙一个出水管，单开甲管20小时可将水池注满水，单开乙管30小时可将水池注满水，单开丙管60小时可将满池水放完．现三管同时打开，多少小时可将水池注满水？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

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

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

<gadget id="calculator">(1/20) + (1/60)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / (1/15)</gadget>
<output>15</output>

<result>15</result>