[tex]\bf \cfrac{4}{5}-k=\cfrac{21}{20}\impliedby
\begin{array}{llll}
\textit{let's multiply both sides by }\stackrel{LCD}{20}\\
\textit{to do away with the denominators}
\end{array}
\\\\\\
20\left( \cfrac{4}{5}-k \right)=20\left( \cfrac{21}{20} \right)\implies 16-20k=21
\\\\\\
16-21=20k\implies -5=20k\implies \cfrac{-5}{20}=k\implies -\cfrac{1}{4}=k[/tex]
