The pool is equipped with a water inlet pipe and a drainage pipe. Open the inlet pipe for 4 minutes to fill the pool, and open the drain pipe for 6 minutes to empty the pool. There is 1/3 dirty water in the pool now. Li Shifu had to drain the dirty water first, but he forgot to turn off the drain when he drained the clean water. How long does it take to fill up the clear water?

Answer: Li Shifu had to drain the dirty water first, but he forgot to turn off the drain when he drained the clean water. Note that he forgot to turn off the drain pipe when he drained the clean water. In other words, he opened the clean water pipe after draining the dirty water, but forgot to close the drain pipe.

The process is as follows, divided into two parts.

The first stage, sewage discharge: there is 1/3 sewage, the drainage pipe is opened for 6 minutes, and then

6* 1/3=2

Discharge the sewage in 2 minutes.

In the second paragraph, put clean water: does water enter every hour? . The drainage rate is per hour 1/6, then

1/(? - 1/6)= 12

/kloc-Fill with clean water in 0/2 minutes.

Then * * * time: 2+ 12= 14.

* * * It takes 14 minutes to add fresh water to the sewage.