9/x = 4/12
cross multiply
4*x = 9*12
x = 9*12/4
x = 9*3
x = 27
Hool
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So, if we change the percentages, to decimal notation, 40 to 40/100 or 0.40 and so on
we get

so.. whatever the addition of those two alloys is, it must equate the mixture of 50% copper, that is

solve for "x"
Cos A would be 4/5, because that is a 3-4-5 right triangle