let's say the total amount for 2011 was "x", so "x" is the 100%.
in 2012 it went down by 10% to 90, so 90 is really 100% - 10%, namely 90 is the 90% of "x".
so since we know that 90 is the 90%, what is "x"?
[tex]\bf \begin{array}{ccll} amount&\%\\ \cline{1-2} x&100\\ 90&90 \end{array}\implies \cfrac{x}{90}=\cfrac{100}{90}\implies \cfrac{x}{90}=\cfrac{10}{9} \\\\\\ 9x=900\implies x=\cfrac{900}{9}\implies \boxed{x=100}[/tex]
