tukaisg
Why we did not use the formula n(A U B U C) = n(A) + n(B) + n(C) - n(A ∩ B) - n(B ∩ C) - n(C ∩ A) + n(A ∩ B ∩ C) + Neither ?
|X ∪ Y ∪ Z| = |X| + |Y| + |Z| - |X ∩ Y| - |Y ∩ Z| - |X ∩ Z| + |X ∩ Y ∩ Z|
28 = 10 + 11 + 13 - 0 - 0 - 0 + a
28 = 34 + a
a = -6
is that why we approached the other formula ?
Hey! Perhaps I can help.
If we were to approach this formulaically, the correct formula is ->
n(A U B U C) = n(A) + n(B) + n(C) - n(A ∩ B) - n(B ∩ C) - n(C ∩ A) - 2 n(A ∩ B ∩ C) Some things to note:(1) n(A U B U C) means every entity (lawyers in this case) belongs to at least one of the 3 groups (A,B,C). There is no "neither" in this case. How can people who belong to none of the 3 groups be part of a group which is about those who belong to one or more of the 3 groups? Hence, the "neither" in your formular shouldn't be there.
(2) n(A) + n(B) + n(C) -> when we do this sum, we count n(A ∩ B), n(B ∩ C), and n(C ∩ A) 2 times. Hence, it has to be removed one to remove duplicate. So "- n(A ∩ B) - n(B ∩ C) - n(C ∩ A)" is correct in your formula.
(3) n(A) + n(B) + n(C) -> when we do this sum, n(A ∩ B ∩ C) gets counted 3 times. So, to remove the additional counting, we have to subtract
2 n(A ∩ B ∩ C). Notice this difference too.
Using the above formula ->n(A U B U C) = n(A) + n(B) + n(C) - n(A ∩ B) - n(B ∩ C) - n(C ∩ A) - 2 n(A ∩ B ∩ C) |X ∪ Y ∪ Z| = |X| + |Y| + |Z| - |X ∩ Y| - |Y ∩ Z| - |X ∩ Z| - 2 |X ∩ Y ∩ Z|
28 = 10 + 11 + 13 - 0 - 0 - 0 -2a
28 = 34 - 2a
2a = 6
a = 3
Only State Z =13 - 3 = 10.
Hope this helps!
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Harsha