A pool full of water has a water inlet valve and three drain valves of the same caliber. If the water inlet valve and a drain valve are opened at the same time, the water in the pool can be drained in 30 minutes; if the water inlet valve and the drain valve are opened at the same time With two drain valves, the water in the pool can be drained in 10 minutes. Q: How many minutes will it take to drain the pool when the inlet valve is closed and three drain valves are opened at the same time?
一个装满了水的水池有一个进水阀及三个口径相同的排水阀，如果同时打开进水阀及一个排水阀，则30分钟能把水池的水排完；如果同时打开进水阀及两个排水阀，则10分钟能把水池的水排完．问：关闭进水阀并且同时开三个排水阀，需要几分钟才能排完水池的水？
<gadget id="calculator">30 / 10</gadget>
<output>3</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">2 / 30</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 3</gadget>
<output>1/5 = around 0.2</output>

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

<result>5</result>