Mixtures and solutions: GRE Math Practice Question#4

 
How many liters of a $12$ percent alcohol solution must be added to $6$ liters of a $24$ percent alcohol solution to obtain a solution that is $15$ percent alcohol?

1. $\quad 9$
2. $\quad 15$
3. $\quad 18$
4. $\quad 20$
5. $\quad 24$