well, the increase is just £28000 - £23500, or £4500.
now, if we take 23500 to be the 100%, what is 4500 off of it in percentage?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
23500&100\\
4500&p
\end{array}\implies \cfrac{23500}{4500}=\cfrac{100}{p}\implies p=\cfrac{4500\cdot 100}{23500}[/tex]
