There are two water pipes A and B of different sizes installed on the pool. Opening pipe A alone can fill the pool in 1 hour. Now the two water pipes can be filled with water at the same time, and the pool can be filled (1/2) in 20 minutes. If pipe B is opened alone, how many minutes will it take to fill the pool?
水池上装有甲乙两个大小不同的水管，单开甲管1小时可注满水池，现在两个水管同时注水，20分钟可注满水池的(1/2)．如果单开乙管，需要多少分钟注满水池．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) / 20</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/40) - (1/60)</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">1 / (1/120)</gadget>
<output>120</output>

<result>120</result>