well, the decrease is 2 bucks, hmmm if we take 20 to be the 100%, how much is 2 off of it in percentage?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 20 & 100\\ 2& x \end{array} \implies \cfrac{20}{2}=\cfrac{100}{x} \\\\\\ 10=\cfrac{100}{x}\implies 10x=100\implies x=\cfrac{100}{10}\implies x=10[/tex]
