Answer:
 2.67 liters
Step-by-step explanation:
Let "a" represent the number of liters of pure acid needed to make the desired solution. Then the amount of acid in the mix is ...
 (100%)x +(10%)(12 -x) = (30%)(12)
 (90%)x = 12(20%) . . . . . subtract (10%)(12)
 x = 12(2/9) . . . . . divide by 90%
 x = 2 2/3 . . . liters
You'll need 2.67 liters of the pure acid.