One water inlet pipe and one water outlet pipe, just open the water inlet pipe for 5 hours to fill a pool of water. After two hours of filling, it is found that the outlet pipe is not closed. After closing the water outlet pipe, continue to pour water into the pool, and it takes another 4 hours to fill the pool. full, how long does it take to open the water pipe to drain the pool of water?
一个进水管和一个出水管，单开进水管5小时就能灌满一池水，在灌水两小时后发现出水管没有关，关闭出水管后再继续向水池灌水，再经4小时才将水池灌满，问开出水管需多少时间才能把一池水放完？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">(1/5) * 6</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">(6/5) - 1</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) / 2</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / (1/10)</gadget>
<output>10</output>

<result>10</result>