In a pool of water, two pipes A and B are opened at the same time, and they are filled in 5 hours, and pipes B and C are opened at the same time, and they are filled in 4 hours. Now open pipe B for 6 hours first, and pipes A and C need to be opened for 2 hours at the same time to be filled. How many hours can B be filled up after being opened alone?
一池水，甲乙两管同时开，5小时注满，乙丙两管同时开，4小时注满．现在先开乙管6小时，还需甲丙两管同时开2小时才能注满．乙单独开几小时可以注满？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">(1/5) + (1/4)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(9/20) * 2</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">6 - 4</gadget>
<output>2</output>

<gadget id="calculator">(1/10) / 2</gadget>
<output>1/20 = around 0.05</output>

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

<result>20</result>