[tex]\bf \begin{array}{lccclll}
&amount&price&\textit{price per amount}\\
&-----&-----&--------\\
\textit{marshmallows}&x&1.80&(1.80)(x)\\
\textit{rice crispies}&y&3.20&(3.20)(y)\\
-----&-----&-----&--------\\
mixture&50&2.92&(50)(2.92)
\end{array}\\\\
-----------------------------\\\\
\begin{cases}
x+y=50\implies \boxed{y}=50-x\\
1.80x+3.20y=(50)(2.92)\\
----------\\
1.80x+3.20\left( \boxed{50-x} \right)=(50)(2.92)
\end{cases}[/tex]
solve for "x", to see how many lbs of marshmallows were needed
what about "y"? well, y = 50 - x
