[tex] A=\{1,3,5,7,9\}\\
B=\{2,4,6,8,10\}\\
C=\{1,5,6,7,9\}\\\\
A \cap (B \cap C)=A \cap B \cap C=\emptyset [/tex]
[tex] A \cap (B \cap C) = [/tex]
[tex] = A \cap (\{2, 4, 6, 8, 10\} \cap \{1, 5, 6, 7, 9\}) [/tex]
[tex] = \{1, 3, 5, 7, 9\} \cap \{6\} [/tex]
[tex] = \{~\} [/tex]