the original price is "x", or 100%.
but we know that if we reduce "x" by 20%, namely 100% - 20% = 80%, the 80% of "x" is really £50, what is "x" or the 100% anyway?
[tex]\bf \begin{array}{ccll} amount&\%\\ \cline{1-2} x&100\\ 50&80 \end{array}\implies \cfrac{x}{50}=\cfrac{100}{80}\implies \cfrac{x}{50}=\cfrac{5}{4}\implies 4x=250 \\\\\\ x=\cfrac{250}{4}\implies x=62.5[/tex]
