A pool is equipped with two water inlet pipes A and B. If the two water inlet pipes are opened together for 12 minutes, the empty pool can be filled, and if the A water pipe is opened alone for 20 minutes, the empty pool can be filled. How many minutes will it take to fill the empty pool with the B water pipe alone?
一个水池装有甲、乙两个进水管，两个进水管一齐打开12分钟可以把空池注满，单开甲水管20分钟可以把空池注满．单开乙水管多少分钟可以把空池注满？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/12) - (1/20)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / (1/30)</gadget>
<output>30</output>

<result>30</result>