if we take the 68.73 to be the 100%, then what is 12 in percentage of that?
since in this case, the tip seems to be a percent of the bill
so [tex]\bf \begin{array}{ccllll}
amount&\%\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\
68.73&100\\
12&x
\end{array}\implies \cfrac{68.73}{12}=\cfrac{100}{x}[/tex]
solve for "x"
