they have "x" in total, now, "x" is the 100%, and we know 28100 is the 31.3%, so.. what's "x"?
well [tex]\bf \begin{array}{ccllll}
amount&\%\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\
x&100\\
28100&31.3
\end{array}\implies \cfrac{x}{28100}=\cfrac{100}{31.3}[/tex]
solve for "x"
