A cistern has two water pipes, the inlet pipe alone can fill the whole pond in 10 minutes, and the outlet pipe can drain the whole pond in 15 minutes. How many minutes will it take to fill the whole pool if the two pipes are opened at the same time?
一个蓄水池有两根水管，单开进水管，10分钟可注满全池，单开出水管15分钟可将全池水放完．两管同时打开，多少分钟可注满全池？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) - (1/15)</gadget>
<output>1/30 = around 0.033333</output>

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

<result>30</result>