Let:
Op = Original price
Sp = Sale price
Express the discount percentage as a decimal:
25% = 25/100 = 0.25
the original price will be given by:
[tex]\begin{gathered} Sp=Op-0.25Op \\ where\colon \\ Sp=90 \\ solve_{\text{ }}for_{\text{ }}Op\colon \\ 90=Op(1-0.25) \\ so\colon \\ Op=\frac{90}{1-0.25} \\ Op=120 \end{gathered}[/tex]$120
