Suppose you pour one liter of alcohol for the first time and (60-a) liters of alcohol, then pour (60-a)×(a+ 14)/60 liters of alcohol for the second time and there are still 30 liters.

Then we can get the equation: 60-a-(60-a) × (a+14)/60 = 30, and a= 10.

Method 2:

Solution: Suppose the first inverted X liter.

Alcohol left over from the first time (60-X)

The concentration after adding water is (60-X)/60.

Second inversion X+ 14

Amount of alcohol poured out (x+ 14)*(60-x)/60.

Pour it out twice (x+ 14)*(60-x)/60+x=30.

(x+ 14)*(60-x)+60x = 1800

x^2- 106x+960=0

Solve the equation, x 1=96, x2= 10.

Clearly only 10.

So it is 10.