Desenam diagrama Venn.
Notam :
[tex]card(M)=x, \ \ card(M\cap N)=y[/tex]
Avem :
[tex]card(M)\cdot card(M-N) = card(M)\cdot [card(M)-card(M\cap N)]= \\\;\\
=x(x-y)=15 \Longrightarrow \begin{cases} x(x-y) = 15\cdot1\\\;\\ x(x-y) = 5\cdot 3\end{cases}
[/tex]