These are my achilles heel...and I plan to get into engineering, so...I really need to get better at these word problems. I am very frustrated.

What quantity of 60% acid solution must be mixed with a 30% solution to produce 300mL of a 50% solution?

What I have tried so far is:

I have 300mL total, of a 30% solution, which means that I currently have 90mL of acid in the 300mL solution.

I need to add x amount of 60% acid solution, which is .6x.

300 + x is what I get when I add the current 300mL to x amount of acid.

It needs to be a 50% solution, so the expression is .5(300 + x) = .3(300) + .6x, right?

I come up with 150mL, but the correct answer is 200mL. What am I missing? I have tried and tried, and even started switching out the numbers, but I cannot come up with the right solution. This is frustrating me to no end...help please !

EDIT: I got it!

Suddenly it just clicked for some reason...I had been trying to solve this for hours.

I remembered that I am adding mixing it with a 30% solution...so that isn't .5(300 + x), it's 150 + .3x, right?

150 + .3x = 90 + .6x

x = 200mL