Question 972639
x of one alloy, y of the other alloy.
Let p, be percentage of copper in the "y" alloy.  The "x" alloy then will have {{{p-(40/100)p=(60/100)p}}} percent copper.


{{{x+y=50}}}, percent copper in x is {{{0.60p}}}.


Determine percent of copper in the first, "x", alloy?
Not enough information for this.  You only can find that the x quantity alloy has {{{0.6p}}} percent copper.


{{{((0.6)p/100)*x=6}}} from the description, "there were 6 kg copper in first alloy";


{{{((0.4p)/100)*y=12}}}, from the other part, "12 kilograms in second alloy"


That showed three equations in three unknown variables.  Solving for x, y, p should be possible.


{{{system(x+y=50,100(0.6p)x=6,100(0.4p)y=12)}}}