A pool is equipped with two water outlet pipes and one water inlet pipe. Opening one water outlet pipe for 40 minutes can discharge a pool of water, and opening one water inlet pipe for 30 minutes can fill a pool of water. Now there is a full pool of water that needs to be drained, because The staff was negligent and opened all three water pipes, asking how long did it take to light the pool of water?
一个水池装有两根出水管和一根进水管，单开一根出水管40分钟可放光一池水，单开一根进水管30分钟可注满一池水，现在有一满池水需放掉，由于工作人员疏忽，把三根水管全打开了，问放光这池水花了多少时间？
<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/40) - (1/30)</gadget>
<output>-1/120 = around -0.008333</output>

<gadget id="calculator">(1/40) + (-1/120)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / (1/60)</gadget>
<output>60</output>

<result>60</result>