set A,B,C have soem elements in common. if 16 elements are in both A and B, 17 elements are in both A and C, and 18 elements are in both B and C, how many elements do all three of the sets A, B, and C have in common?
1) of the 16 elements that are in both A and B, 9 elements are also in C
2) A has 25 elements, B has 30 elements, and C has 35 elements.
the answer is A. why? | 
