if we take 12 to be the 100%, now 15 - 12, so is 3 bucks up for the retail price, what is 3 off 12 in percentage then?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
12&100\\
3&p
\end{array}\implies \cfrac{12}{3}=\cfrac{100}{p}\implies p=\cfrac{3\cdot 100}{12}[/tex]
