A metalworker has a metal alloy that is 30% copper and another alloy that is 75% copper. How many kilograms of each alloy should the metalworker combine to create 120 kg of a 57% copper alloy?
Let x be the number of kilograms of the 30% copper alloy
Thus, 30% * x + 75% * (120 kg - x) = 57% * 120 kg
Simplifying the equation gives 30% * x + 75% * 120 kg - 75% * x = 57% * 120 kg
Combining like terms gives 45% * x = 57% * 120 kg - 75% * 120 kg
Simplifying the right side of the equation gives 45% * x = 57% * 120 kg - 75% * 120 kg
Simplifying the equation gives 45% * x = 20% * 120 kg
Dividing both sides of the equation by 45% gives x = (20% * 120 kg) / 45%
Thus, x = 0.2 * 120 kg / 0.45 = 24 kg of the 30% copper alloy
Thus, the metalworker should combine 24 kg of the 30% copper alloy with 120 kg - 24 kg = 96 kg of the 75% copper alloy. Answer: \boxed{24}.