There is 16 tons of water in a pool. The pool is equipped with two drainage pipes A and B and water inlet pipe C. Pipe A alone discharges 5 tons per hour, pipe B alone discharges 1 ton per hour, and pipe C alone drains water every hour. Inlet water 4 tons. Now, according to the order of A, B, C, A, B, C... each open for 1 hour in turn. Ask how many hours have elapsed before the water in the pool has just been emptied.
一水池内有16吨水，水池装有A、B两个排水管和C进水管，A管独开每小时排水5吨，B管独开每小时排水1吨，C管独开每小时灌进水4吨．现在按A、B、C、A、B、C…的顺序轮流各开1小时．问经过多少小时，水池的水刚好排空．
<gadget id="calculator">1 - 4</gadget>
<output>-3</output>

<gadget id="calculator">5 + (-3)</gadget>
<output>2</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">15 + 2</gadget>
<output>17</output>

<result>17</result>