20L pure alcohol, pour out a part and fill it with water. Pour out the same amount of mixed liquid as before for the second time, and then fill it with water. At this time, the water in the container is three times that of pure alcohol. Find the amount of alcohol poured out for the first time.

Let x pour wine for the first time.

The final alcohol concentration is: 1/( 1+3)=25%.

The pure alcohol poured out for the second time is 20-20*25%.

x+[(20-x)/20]*x=20-20*25%，

Solution: x= 10, x=30 (not in line with the meaning of the question)

That is, the first pouring 10 liter.