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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 ?
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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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hi Bunuel do you have more questions like these please? triple/overlapping sets. thanks!
Bunuel
A certain law firm has 28 lawyers and has offices in three states, X, Y, and Z. Every lawyer in the firm is either licensed in all three states or is licensed in only one of the three states. If 10 lawyers are licensed in State X, 11 are licensed in State Y, and 13 are licensed in State Z, how many lawyers are licensed only in State Z ?

A. 3
B. 7
C. 8
D. 10
E. 25
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hi Bunuel do you have more questions like these please? triple/overlapping sets. thanks!
Bunuel
A certain law firm has 28 lawyers and has offices in three states, X, Y, and Z. Every lawyer in the firm is either licensed in all three states or is licensed in only one of the three states. If 10 lawyers are licensed in State X, 11 are licensed in State Y, and 13 are licensed in State Z, how many lawyers are licensed only in State Z ?

A. 3
B. 7
C. 8
D. 10
E. 25

You can search in overlapping sets: https://gmatclub.com/forum/search.php?s ... &tag_id=65
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KarishmaB Bunuel Could you please help why we didn't use first formula (Total=A+B+C−(sum of 2−group overlaps)+(all three)+Neither.)?
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KarishmaB Bunuel Could you please help why we didn't use first formula (Total=A+B+C−(sum of 2−group overlaps)+(all three)+Neither.)?

Check ADVANCED OVERLAPPING SETS PROBLEMS
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IMO easiest way:

10+11+13 = 34 licences for 28 lawyers.

If each lawyer has one license each, theres 34-28 = 6 licenses left.

Since no lawyer has 2 licenses, there are some amount of lawyers with three licenses that take up the extra 6 left.

Since each lawyer has one license already, theres some amount of lawyers with an extra two licenses. 6/2 = 3 lawyers have 3 licenses i.e.

25 lawyers with one licence.
3 lawyers with three licenses.

Z only = 13-3 = 10.
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