Question 1206112
One can transcribe the description exactly as written, 
here using w for the whole quantity of mixture.
<pre>
Copper          (w/2)-16
Tin             (w/3)-12
Lead            (w/4)+4
WHOLE             w
</pre>

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{{{w=288}}}


Another way,  the whole mixture is some 12 equal parts, so 12M is this mixture quantity.
<pre>
Copper       6M-16
Tin          4M-12
Lead         3M+4
WHOLE        12M
</pre>
Remember M here is a factor.
Equation to solve can be  {{{(6M-16)+(4M-12)+(3M+4)=12M}}}.

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