"A metalworker has a metal alloy that is 15% copper and another alloy that is 60% copper. How many kilograms of each alloy should the metalworker combine to create a 80 kg of a 42% copper alloy?"

Let

x = kg of the 15% copper alloy

y = kg of the 60% copper alloy

Now we can write the following system of equations:

x + y = 80

0.15x + 0.60y = 0.42*80

........

click here to see the system of equations solved for x and y

........

x = 32 kg

y = 48 kg

32 kg of the 15% alloy and 48 kg of the 60% alloy should be combined to make 80 kg of 42% alloy.