well, we can say that 1 hour is 100%, what is 2/5 hr off of it in percentage?
[tex]\begin{array}{ccll} hour&\%\\ \cline{1-2} 1 & 100\\ \frac{2}{5}& x \end{array} \implies \cfrac{1}{~~ \frac{2}{5}~~}=\cfrac{100}{x}\implies \cfrac{5}{2}=\cfrac{100}{x} \\\\\\ 5x=200\implies x=\cfrac{200}{5}\implies x=40[/tex]
