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Let $f$ be a linear function for which $f(5)-f(2)=10$. What is $f(8)-f(2)$? (  Adapted From 2003 AMC 12B Problems, Question \#9) 
Overseas Competition->Knowledge Point->Calculation Modules
Let $f$ be a linear function with slope $m$.  $$ \begin{gathered} m=\frac{f(5)-f(2)}{\Delta x}=\frac{10}{5-2}=\frac{10}{3} \textbackslash\textbackslash{} f(8)-f(2)=m \Delta x=\frac{10}{3}(8-2)=20 \Rightarrow(C) \end{gathered}$$ 