pool has an inlet pipe and a drain pipe. Opening the water inlet pipe alone can fill the pool for 40 minutes, and opening the drain pipe alone can drain the full pool of water in 1 hour. There was originally (1/3) dirty water in the pool, Master Wang had to drain the dirty water first, and then put half of the pool with clean water. But he forgot to close the drainpipe when putting in clean water. So how many minutes will it take to fill half a pool of clean water?
一个水池装有一支进水管和一支排水管．单开进水管40分钟可以把水池注满，单开排水管1小时可以把满池水排完．池内原有(1/3)的脏水，王师傅要先排尽脏水，再放进半池清水．但是放进清水时他忘记关闭排水管．那么要用多少分钟才能放满半池清水？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/40) - (1/60)</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">(1/2) / (1/120)</gadget>
<output>60</output>

<result>60</result>