A pool is equipped with two water pipes, A and B. When pipe A is turned on alone, the water in the pool can be completely drained in 4 hours; if pipe B is only turned on, it can be filled with water in 8 hours. Now that the pool is full, turn on A, B, A, B... for 1 hour each. After how many hours, the pool will be empty for the first time.
一个水池装有甲乙两水管，单开甲管4小时能把满池水排完；只开乙管8小时能灌满一池水．现水池是满的，按甲、乙、甲、乙…轮流各开1小时，多少小时后水池第一次没有水．
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(1/4) - (1/8)</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(3/4) / (1/8)</gadget>
<output>6</output>

<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<gadget id="calculator">1 + 12</gadget>
<output>13</output>

<result>13</result>