let us convert the percentages to decimal format first.. so 5% is just 5/100 or 0.05 and 15% is just 15/100 or 0.15
so hmmm, so, let's say it needs "x" amount and "y" amount of each respectively, so, whatever "x" and "y" are, they must add up to 100, and whatever their concentration is, must add up to what the mixture yields
thus
[tex]\bf \begin{array}{lccclll}
&amount&concentration&
\begin{array}{llll}
concentrated\\
amount
\end{array}\\
&-----&-------&-------\\
\textit{5\% alloy}&x&0.05&0.05x\\
\textit{30\% alloy}&y&0.30&0.3y\\
-----&-----&-------&-------\\
mixture&100&0.15&15
\end{array}
\\\\\\
\begin{cases}
x+y=100\implies \boxed{y}=100-x\\
0.05x+0.3y=15\\
----------\\
0.5x+0.3\left( \boxed{100-x} \right)=15
\end{cases}[/tex]
solve for "x"
what's "y"? well, y = 100 - x
