[tex]\bf \begin{array}{ccllll}
\textit{cups of milk}&serving\\
-----&-----\\
\frac{3}{4}&1\\
x&20
\end{array}\implies \cfrac{\frac{3}{4}}{x}=\cfrac{1}{20}\implies \cfrac{\frac{3}{4}}{\frac{x}{1}}=\cfrac{1}{20}
\\\\\\
\cfrac{3}{4}\cdot \cfrac{1}{x}=\cfrac{1}{20}\implies \cfrac{3}{4x}=\cfrac{1}{20}\implies 3\cdot 20=1\cdot 4x\implies \cfrac{3\cdot 20}{4}=x[/tex]
and surely you know how much that is.
