well, from 40 to 28, the discount is clearly 12 bucks.
if we take 28 to be the 100%, what is 12 off of it in percentage?
[tex]\bf \begin{array}{ccll} amount&\%\\ \cline{1-2} 28&100\\ 12&x \end{array}\implies \cfrac{28}{12}=\cfrac{100}{x}\implies \cfrac{7}{3}=\cfrac{100}{x} \\\\\\ 7x=300\implies x=\cfrac{300}{7}\implies x\approx 42.86[/tex]
