A metalworker has a metal alloy that is 25% copper and another alloy that is 75% copper. How many kilograms of each alloy should the metalworker combine to create 60kg of a 65% copper alloy?
Let x be the amount of the 25% copper alloy.
Let y be the amount of the 75% copper alloy.
We know that x + y = 60 (since the total weight of the alloys is 60kg).
We also know that 0.25x + 0.75y = 0.65(60) (since the total weight of the copper in the final alloy is 65% of 60kg).
Rewriting the first equation, we have x = 60 - y.
Substituting this into the second equation, we get 0.25(60 - y) + 0.75y = 39.
Expanding and combining like terms, we get 15 - 0.25y + 0.75y = 39.
Combining like terms, we get 0.50y = 24.
Dividing both sides by 0.50, we get y = 48.
Substituting this value into the equation x = 60 - y, we get x = 60 - 48 = 12.
Thus, the metalworker should combine 12kg of the 25% copper alloy with 48kg of the 75% copper alloy to create 60kg of a 65% copper alloy. Answer: \boxed{12}.