[tex]\bf \begin{array}{lccclll}
&amount&price&priced\ amount\\
&-----&-----&--------\\
\textit{9 bucks/lb}&x&9&9x\\
\textit{3 bucks/lb}&y&3&3y\\
-----&-----&-----&-------\\
mixture&240&5&1200
\end{array}
\\\\\\
\begin{cases}
x+y=240\implies \boxed{y}=240-x\\
9x+3y=1200\\
----------\\
9x+3\left( \boxed{240-x} \right)=1200
\end{cases}[/tex]
solve for "x".
what about "y"? well, y = 240 - x