# Math Help - Number of triples

1. ## Number of triples

Prove that the number of triples $(A, B, C)$ where $A, B, C$ are subsets of ${1, 2, ..., n}$ such that $A \cap B \cap C = \phi, A \cap B \ne \phi, B \cap C\ne \phi$ is $7^n - 2.6^n + 5^n$.