A reservoir has two water inlet pipes AB, if both pipes are opened at the same time for 12 hours, the empty pool can be filled. If pipe A is opened for 5 hours first, and then pipe B is opened, it will take another 9 hours to fill the empty pool. How many hours does it take to fill the empty pool with only A pipe?
一个蓄水池有AB两个进水管，如果同时开两管12小时可以将空池注满．如果先开A管5小时，再才打开B管，又要9小时，也可将空池注满．单开A管多少小时将空池注满？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 9</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) / 5</gadget>
<output>1/20 = around 0.05</output>

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

<result>20</result>