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find the intersection value of 3 subsets

schinb65

New member
Jan 1, 2013
12
Let a, b and c be three subsets of universe U with the following properties: n(A)= 63, n(B)=91, n(c)=44, The intersection of (A&B)= 25, The intersection of (A&C)=23, The intersection of (C&B)=21, n(A U B U C)= 139. Find the intersection of (A&B&C).

I am told the answer is 10.


I tried drawing a diagram.

A: B: C:
x x x
25-x 21-x 23-x
23-x 25-x 21-x
63-(x + 21-x + 23-x) 91-(x+25-x+21-x) 44-(x+23-x+21-x)


I added all of these together and had them equal to 198, since I am using values repeatedly. I also omitted the repeats and set the sum to 139. This should give me the correct answer correct?
 

caffeinemachine

Well-known member
MHB Math Scholar
Mar 10, 2012
834
Let a, b and c be three subsets of universe U with the following properties: n(A)= 63, n(B)=91, n(c)=44, The intersection of (A&B)= 25, The intersection of (A&C)=23, The intersection of (C&B)=21, n(A U B U C)= 139. Find the intersection of (A&B&C).

I am told the answer is 10.


I tried drawing a diagram.

A: B: C:
x x x
25-x 21-x 23-x
23-x 25-x 21-x
63-(x + 21-x + 23-x) 91-(x+25-x+21-x) 44-(x+23-x+21-x)


I added all of these together and had them equal to 198, since I am using values repeatedly. I also omitted the repeats and set the sum to 139. This should give me the correct answer correct?
What you have written in the last paragraph is essentially what you need to do. I don't understand the rest of your attempt though but I think you are on the right track. You might find this useful Inclusion
 

soroban

Well-known member
Feb 2, 2012
409
Hello, schinb65!

Let $A, B, C$ be three subsets of universe $U$ with the following properties:

. . $\begin{array}{c}n(A)\,=\, 63 \\ n(B)\,=\,91 \\ n(C)\,=\,44\end{array} \qquad \begin{array}{c} n(A\cap B) \,=\,25 \\ n(B\cap C) \,=\,21 \\ n(A\cap C) \,=\, 23 \end{array} \qquad n(A\cup B \cup C) \,=\,139 $

Find: $ n(A \cap B\cap C)$

Are you familiar with this formula?

$n(A \cup B \cup C) \:=\:n(A) + n(B) + n(C) $

. . . . . . . . . . . . . . $+\,n(A \cap B) + n(B \cap C) + n(A \cap C) $

. . . . . . . . . . . . . . . . $+ n(A \cap B \cap C)$
 

Jameson

Administrator
Staff member
Jan 26, 2012
4,043
Hello, schinb65!


Are you familiar with this formula?

$n(A \cup B \cup C) \:=\:n(A) + n(B) + n(C) $

. . . . . . . . . . . . . . $+\,n(A \cap B) + n(B \cap C) + n(A \cap C) $

. . . . . . . . . . . . . . . . $+ n(A \cap B \cap C)$
Hi soroban,

I think what you meant was this:
$n(A \cup B \cup C) \:=\:n(A) + n(B) + n(C) -\,n(A \cap B) - n(B \cap C) - n(A \cap C) + n(A \cap B \cap C)$.

Just a typo I think :)

Jameson