if we take 800 to be the 100%, and the difference is just 100 bucks, how much is 100 off of it in percentage?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
800&100\\
100&p
\end{array}\implies \cfrac{800}{100}=\cfrac{100}{p}\implies p=\cfrac{100\cdot 100}{800}[/tex]
