A cistern is equipped with two water pipes A and B, first open pipe A to fill water for 4 hours, then close pipe A, then open pipe B to fill water for 16 hours, just to fill the pool; if first open pipe B to fill water for 4 hours, then close pipe B , and then open a pipe to inject water for 10 hours, it just fills the pool. How many hours can the pool be filled with only pipe A?
一个蓄水池装有甲、乙两条水管，先开甲管注水4小时，关上甲管，再开乙管注水16小时，正好注满水池；如果先开乙管注水4小时，关上乙管，再开甲管注水10小时，也正好注满水池．只开甲管多少小时能注满水池？
<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<gadget id="calculator">10 - 4</gadget>
<output>6</output>

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

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<gadget id="calculator">4 + 8</gadget>
<output>12</output>

<result>12</result>