B
1) To solve this question, we can write out two equations within this expression:
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:
3) So we found 20l of the 15% solution.
Thus the answer is B