There are two water pipes A and B installed on the pool. If both pipes are opened together for 12 hours, the pool will be filled. If pipe A is opened for 6 hours and pipe B is opened for 5 hours, only 9/20 of the volume of the pool will be filled. hours done?
水池上装有甲、乙两根水管，齐开两水管12小时注满水池，若甲管开6小时，乙管开5小时只注了水池容积的(9/20)，那么单开乙管几小时完成？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 6</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">9 / 20</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(1/2) - (9/20)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / (1/20)</gadget>
<output>20</output>

<result>20</result>