Ametalworker has a metal alloy that is 15% copper and another alloy that is
75% copper. how many kilograms of each alloy should the metalworker combine to create 120 kg of a 51% copper alloy?

how do i set up this equation?

Let's say x = 15% copper and y = 75% copper
We need 120 kg of a 51% copper alloy.
A) x + y = 120
B) .15x + .75y = 61.2 (which is 120*.51%)
Multiplying A by -.15
A) -.15x  -.15y = -18 Then add this to B
.60y = 43.2
y = 72
x = 48
I'd suggest you look at this web page
http://www.1728.org/mixture.htm

Double Check
48 kilograms of .15% copper = 7.2 kg copper
72 kilograms of .75% copper = 54 kg kg copper
So we have 61.2 kg of copper in 120 kg
61.2 / 120 = .51 = 51%