Question 353765
Let {{{X}}} be the amount of alloy A, {{{Y}}} alloy B, {{{Z}}} alloy C.
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Lead:{{{40X+20Y+0Z=15.2(X+Y+Z)=15.2(325)=4940}}}
Zinc:{{{30X+30Y+10Z=37.2(325)=12090}}}
Copper:{{{30X+50Y+90Z=47.6(325)=15470}}}
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{{{(matrix(3,3,40,20,0,30,30,10,30,50,90))*(matrix(3,1,X,Y,Z))=(matrix(3,1,4940,12090,15470))}}}
Using matrix inversion,
{{{(matrix(3,1,X,Y,Z))=(matrix(3,3,0.055,-0.045,0.005,-0.06,0.09,-0.01,0.015,-0.035,0.015))*(matrix(3,1,4940,12090,15470))}}}
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{{{(matrix(3,1,X,Y,Z))=(matrix(3,1,-195,637,-117))}}}
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I get the same answer. 
Something is up with the problem setup.