so, if we change the percentages, to decimal notation, 40 to 40/100 or 0.40 and so on
we get [tex]\bf \begin{array}{llll}
&quantity&\textit{copper \%}&\textit{copper quantity}\\
&------&------&------\\
\textit{40\% alloy}&x&0.40&(x)(0.40)\\
\textit{60\% alloy}&500&0.60&(500)(0.60)\\
------&------&------&------\\
mixture&x+500&0.50&(x+500)(0.50)
\end{array}[/tex]
so.. whatever the addition of those two alloys is, it must equate the mixture of 50% copper, that is [tex]\bf (x)(0.40)+(500)(0.60)=(x+500)(0.50)[/tex]
solve for "x"
