well, he paid 20.51 total, but the retail price is just 18.99, so he ended up paying 20.51 - 18.99 extra, or 1.52.
so, if we take 18.99 to be the 100%, how much is 1.52 in percentage off of it?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
18.99&100\\
1.52&t
\end{array}\implies \cfrac{18.99}{1.52}=\cfrac{100}{t}\implies t=\cfrac{1.52\cdot 100}{18.99}[/tex]
