$$75$$ out of $$100$$ apples are \textbf{not} rotten. What percentage of the apples are rotten? 
Overseas Competition->Knowledge Point->Word Problem Modules->Solving Problems Involving Fractions and Percentages->Finding the Rate
$$\frac{75}{100}=75 \textbackslash\%$$,  $$100\textbackslash\%-75\textbackslash\%=25\textbackslash\%$$,  $$25\textbackslash\%$$ of the apples are rotten. 