Question 25568
You start with 12 quarts; you add "x" quarts; you end up with "12+x" quarts.
Now follow the alcohol:
 10%(12)+100%(x)=20%(12+x)
Multiply thru by 10 to get:
  (12)+10(x)=2(12+x)
  12+10x=24+2x
    8x=12
     x=1 1/2 quarts

Follow this same pattern to determine the number of quarts of 
100% alcohol to get a 25% solution.

Cheers,
Stan H.