well we know that we have 80mL and we also know that those 80mL will represent the "solute" or the 40% of the Tennessee whiskey total which we can say is 100%, so if 80 is the 40%, how much will it be for the 100% in mL?
[tex]\begin{array}{ccll}mL&\%\\\cline{1-2}80&40\\x&100\end{array}\implies \cfrac{80}{x}=\cfrac{40}{100}\implies \cfrac{80}{x}=\cfrac{2}{5}\implies 400=2x\\\\\\ \cfrac{400}{2}=x\implies \underset{mL}{200}=x[/tex]
