Before going to vacation, each of the $ 7$ pupils decided to send to each of the $ 3$ classmates one postcard. Is it possible that each student receives postcards only from the classmates he has sent postcards?
Source: Moldova NMO 2002 grade 7 problem nr.1
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Before going to vacation, each of the $ 7$ pupils decided to send to each of the $ 3$ classmates one postcard. Is it possible that each student receives postcards only from the classmates he has sent postcards?