Seems like it's easier just to solve it straight.
(30%+50%)/2=40%. So the amount replaced was 50-30 or 20%.
Or am I missing something?
I don't think this is correct.
You have a 50% solution and a 30% solution. You are replacing part of the 50% solution with the 30% solution to make a 40% solution.
There are lots of different approaches to solve the problem.
Intuitively -- you know that 40% is in between 30% and 50% -- so 1/2 of the 50% solution will need to be replaced.
You can use algebra and solve this or use the tables. You can also use the "allegation" method -- which is quite useful in these problems. I don't quite follow how you got 20%.