if we take 54 to be the reduced price, and "x" to be the original price, now "x" is the 100%, and 54 is the 100% - 10%, or 90%.
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
x&100\\
54&90
\end{array}\implies \cfrac{x}{54}=\cfrac{100}{90}\implies x=\cfrac{54\cdot 100}{90}[/tex]
