There are two faucets A and B on the pool. Turning on faucet A alone can fill the pool in 2 hours, and turning on faucet B alone can empty the pool in 4 hours. When both faucets A and B are turned on at the same time, how many hours can the pool be filled?
水池上装有甲、乙两个水龙头，单开甲水龙头2小时可将水池注满，单开乙水龙头4小时可排空一池水，甲、乙两个水龙头同时打开，几小时可注满水池？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/2) - (1/4)</gadget>
<output>1/4 = around 0.25</output>

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

<result>4</result>