well, the loss is clearly 10 bucks, if we take 200 to be the 100%, how much is 10 off of it in percentage?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 200&100\\ 10&x \end{array}\implies \cfrac{200}{10}=\cfrac{100}{x}\implies 20=\cfrac{100}{x} \\\\\\ 20x=100\implies x=\cfrac{100}{20}\implies x = 5[/tex]
