Two water pipes, A and B, inject water into the pool. When pipe A is opened alone for 20 hours, a pool of water can be filled, and when pipe B is opened alone for 40 hours, a pool of water can be filled. After the two pipes are opened together, water is injected into the pool for 8 hours, and then A alone fills the pool with water. How long will it take for pipe A to fill the pool?
甲、乙两水管往水池中注水，甲管单独打开20h可注满一池水，乙管单独打开40h可注满一池水，两管一起打开往水池中注水8h后，再由甲单独注水，则甲水管还需多长时间可注满水池？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(1/20) + (1/40)</gadget>
<output>3/40 = around 0.075</output>

<gadget id="calculator">(3/40) * 8</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) / (1/20)</gadget>
<output>8</output>

<result>8</result>