she saved $3, and we know the discount is 15%, so her savings is that 15%, thus we know 15% of that purchase is 3.
let's say the full price is x, and therefore the 100%, if we know 3 is the 15%, what is x?
[tex] \bf \begin{array}{ccll}
amount&\%\\
\cline{1-2}
3&15\\
x&100
\end{array}\implies \cfrac{3}{x}=\cfrac{15}{100}\implies \cfrac{3\cdot 100}{15}=x\implies 20=x [/tex]
