Georges, Patrick, Charlis, Samuel and Bruno are five Frenchmen. Georges knows English and Chinese. Patrick knows Chinese and Japanese. Charlis knows Japanese and German. Samuel knows German and English. Bruno knows them all: English, Chinese, Japanese and German.
In how many ways can they sit in a row, such that any two neighbouring persons have no problems in communication?
Answer
54321=120.
They are Frenchmen, and can all speak French. Therefore, any order is OK.
