Answer
$3\frac{3}{7}\ L$
Work Step by Step
Step 1. Assume we need $x\ L$ of pure alcohol, the total volume would be $x+12$
Step 2. The alcohol before mix is $x(1)+12(0.1)=x+1.2$
Step 3. The alcohol after mix is $(x+12)(0.3)=0.3x+3.6$
Step 4. Let $x+1.2=0.3x+3.6$, we have $0.7x=2.4$, thus $x=\frac{24}{7}=3\frac{3}{7}\ L$