well, the difference is £20 - £3 simply £17.
now, if we take £20 to be the 100%, what is £17 off of it in percentage?
[tex]\begin{array}{ccll} \pounds &\%\\ \cline{1-2} 20 & 100\\ 17& x \end{array} \implies \cfrac{20}{17}=\cfrac{100}{x}\implies 20x=1700 \\\\\\ x=\cfrac{1700}{20}\implies x=85[/tex]
