Calculating Percent for a Lab Experiment, how do you solve?

My word problem is:



How much of an alloy that is 20% copper should be mixed with 200 ounces of an alloy that is 50% copper in order to get an alloy that is 30% copper?



Here is a few equations that I tried, that didn't work.

.20x+.50x=.30

.20x+.50(x-200)=.30

.20x+.50(200-x)=.30



Basically, I just don't understand how to set any of these "acid solution" type problems up, and my Pearson Prentice Hall book is absolutely the worst math book I've ever tried to read and comprehend.



Thank you in advance for your help!Calculating Percent for a Lab Experiment, how do you solve?
it would be

.2x+.5x(200)=.3(x+200)Calculating Percent for a Lab Experiment, how do you solve?
Let x be the unknown mass of the 20% copper alloy. You know the mass of the 50% copper alloy is 200 oz. And the total mass is then x + 200 oz. So...

0.2x + .5*200 oz = 0.3(x + 200 oz)

0.2x + 100 oz. = 0.3x + 60 oz

-0.1 x = -40 oz

x = 400 oz