let's say the original price was "x", which is the 100%, we know she paid $20, but is discounted by 30%, so the 20 bucks is really 100% - 30% or 70%.
so we know $20 is 70%, what is "x" anyway?
[tex]\bf \begin{array}{ccll}
\${data-answer}amp;\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
20&70\\
x&100
\end{array}\implies \cfrac{20}{x}=\cfrac{70}{100}\implies \cfrac{20\cdot 100}{70}=x[/tex]