Not sure if right board, but it was on a pre-calc test...

I expect this post to be moved if the problem seems inappropriate and I apologize in advance. I saw this question on a pre-calc test and it stumped me. I'm just not sure how to set up the equation, I guess. Here it goes:

If 100 liters of Solution A, with 7% salt, is going to be mixed with Solution B, that has 20% salt, in order to create a Mixture C with 15% salt, what is the amount of liters that is needed of Solution B?

So, I have

100 liters of solution A (7% salt)

X liters of Solution B (20% salt)

100+x liters of Mixture C (15% salt)

Is this the correct way to go about it:

0.07(100) + 0.2(x) = 0.15(x+100) ?

Re: Not sure if right board, but it was on a pre-calc test...