100% of the cost is $85. To find how much 20% represent, we can use the next proportion:
[tex]\frac{100\text{ \%}}{20\text{ \%}}=\frac{85\text{ \$}}{x\text{ \$}}[/tex]Solving for x,
[tex]\begin{gathered} 100\cdot x=85\cdot20 \\ x=\frac{1700}{100} \\ x=17\text{ \$} \end{gathered}[/tex]Then, a discount of $17 must be applied. The cost after applying the discount is $85 - $17 = $68
