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