[tex] \left|\begin{array}{ccc}280&-&100\%\\210&-&p\%\end{array}\right|\ \ \ \ |cross\ multiply\\\\\\p\cdot280=210\cdot100\\280p=21000\ \ \ |divide\ both\ sides\ by\ 280\\\boxed{p=75\ (\%)}[/tex]
[tex]Answer:\ 100\%-75\%=\boxed{25\%}[/tex]
Other solution:
[tex]280-210=70\\\\\frac{70}{280}=\frac{70:70}{280:70}=\frac{1}{4}\\\\a=a\cdot100\%\to\frac{1}{4}=\frac{1}{4}\cdot100\%=\frac{100}{4}\%=\boxed{25\%}[/tex]
