well, the decrease was 130 - 39, that is 91
so... if 130 is the 100%, what is 91 of that in percentage?
well \(\bf \begin{array}{ccllll}
amount&\%\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\
130&100\\
91&x
\end{array}\implies \cfrac{130}{91}=\cfrac{100}{x}\)
solve for "x"
