Mixing Acids

First figure out how much acid is currently in the 40% solution. Then figure out how much liquid would be in a 30% solution with that much acid. It will be a number greater than 20. The difference between it and 20 is what you have to add.

Yes
I can probably solve this question by settting up a proportion.
Yes or no?

20 ounces/x = 40%/30% and then solve for x. Is this correct?

Can I use this equation?

Close, but you've got it backwards as you'll discover when you get an answer > 20. What you want is 20*40% = x*30%.

Okay
I see what's going on now. I had it backwards.