Let A, B and C be three subsets of a universe U with the following properties:
n( A ) = 78, n( B ) = 73, n( C ) = 91,
n( A $\displaystyle \cap$ B ) = 16, n( A $\displaystyle \cap$ C ) = 15, n( B $\displaystyle \cap$ C ) = 40
n( A $\displaystyle \cup$ B $\displaystyle \cup$ C ) = 175
Find n( A $\displaystyle \cap$ B $\displaystyle \cap$ C ).