其它
Dexter sprinted for the first $600\textasciitilde m$ of a race in $160\textasciitilde s$ and jogged the remaining $\frac{2}{5}$ of the race in $240\textasciitilde s$. What was his average speed? 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle
Average speed $=\frac{Total distance}{Total Time}=\frac{1000\textasciitilde m}{400\textasciitilde s}=2.5\textasciitilde m/s$ 