There are many ways to solve a problem. That should be enough to make the OP happy!

]]>Nice and compact, Bobby! I didn't think of going that way.

This is how I've done it:

Initially...

Copper = 20% of 200 lbs = 40 lbs

Magnesium = 40% of 200 lbs = 80 lbs

Other = 40% of 200 lbs = 80 lbs

Totals = 100% = 200 lbs

When added...

Copper = 24% of x lbs

Magnesium = 40% of x lbs

Other = 26% of x lbs, ∴ x = 80/26% lbs

Total = 100%

Copper added = 24% of x = (24*80/26)-40 lbs = (960/13)-40 = 440/13

Magnesium added = 50% of x = (50*80/26)-80 lbs = (2000/13)-80 = 960/13

Let c be the amount of copper to be added and m the amount of magnesium to be added

c = 440 / 13, m = 960 / 13

]]>Sorry, but I'm tied up doing something else for quite a while, and so I can only give a quick answer that isn't exact (it's a decimal approximation), and as I got it with Excel's Goal Seek I can't give the method you probably want, either.

Here's what I got:

Copper: add 33.846153846154 lbs

Magnesium: add 73.846153846154 lbs