B
1) To solve this question, we can write out two equations within this expression:
[tex]\begin{gathered} (x)30+(30-x)15=30(20) \\ 30x+450-15x=600 \\ 15x=600-450 \\ 15x=150 \\ \frac{15x}{15}=\frac{150}{15} \\ x=10 \end{gathered}[/tex]
So we can do it with 10L of that 30% solution.
2)Let's find the other volume, plugging into that x=10, since the whole volume of this new solution is 30L we can subtract from that 30L that 10L we have just found:
[tex]30-10=20l[/tex]
3) So we found 20l of the 15% solution.
Thus the answer is B
