Paul had eight $$\textbackslash$5$$-notes at first. He exchanged all his money for $$\textbackslash$2$$-notes only. How many notes did he have in the end? 
Overseas Competition->Knowledge Point->Word Problem Modules->Unitary Method Problems->Unitary Method with One Variable
He has $=\textbackslash$5\times8=\textbackslash$40$  Number of $\textbackslash$2$-notes $=\textbackslash$40\div\textbackslash$2=20$ 