Answer:
The metalworker should use 60 kg of metal alloy that is 70% copper and 40 kg of metal alloy that is 20% copper.
Explanation:
System of Equations
Let's call
x=amount of metal alloy that is 20% copper
y=amount of metal alloy that is 70% copper
The metalworker wants to create 100 Kg of 50% copper alloy. This gives us the first relation
x+y=100
The combination of 20% of x and 70% of y will produce 50% of the sum of both, thus
20x+70y=50(x+y)
Operating
20x+70y=50x+50y
Reducing and dividing by 10
2y=3x
The system of equations is easily solved by isolating x in the first equation
x=100-y
And replacing into the last equation
2y=3(100-y)
Operating
2y=300-3y
5y=300
y=60
Thus
x=100-60=40
The metalworker should use 60 kg of metal alloy that is 70% copper and 40 kg of metal alloy that is 20% copper.