so Ms. Davis gets hmmm say "x" percent from sales as her commission, and we know she did in sales $309 and made $27.81.
if we take 309 to be the 100%, and x% of that is 27.81, what's "x"?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 309 & 100\\ 27.81& x \end{array} \implies \cfrac{309}{27.81}=\cfrac{100}{x} \\\\\\ 309x=2781\implies x=\cfrac{2781}{309}\implies x=9[/tex]