Ametalworker has a metal alloy that is 15% copper and another alloy that is 55% copper. how many kilograms of each alloy should the metalworker combine to create 120 kg of a 39% copper alloy?

               Qty   ·   %      =      Total
Alloy 1:     x     ·  15%   =     .15x
Alloy 2: 120-x  · 55%   =      .55(120 - x) → 66 - .55x
Mixture:  120   · 39%   =      .15x + 66 - .55x

               120   · .39   =     66 - .40x
                     46.8      =     66 - .40x
                    -19.2      =          - .40x
                       48       =      x
Alloy 1:x = 48
Alloy 2: 120 - x = 120 - 48 = 72

The metalworker should use 48 kg of the 15% copper and 72 kg of the 55% copper.