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## #1 2007-02-28 07:32:24

mathsyperson
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Registered: 2005-06-22
Posts: 4,900

### Question by antgaudi

This post was originally posted in an existing topic in the Formulas section, so it has been moved here.

In the future, please create a new topic in the Help Me! section whenever you have a question that you would like help with. Hopefully you'll find this.

antgaudi wrote:

If I have:
U= 200
n(A)=100
n(B)=80
n(A intersects B)=40

How can I obtain n(A complement which intersects B)?

Thanks.

Why did the vector cross the road?
It wanted to be normal.

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## #2 2007-02-28 08:23:32

John E. Franklin
Member
Registered: 2005-08-29
Posts: 3,588

### Re: Question by antgaudi

Just a guess, but

This assumes that U is everything.
And 60 units are outside of A and B.
And 60 units are outside of AB and in A.
And 40 units are outside of AB and in B.
And 40 units are in both A and B, which is AB.
So 200 = 60 + 60 + 40 + 40

igloo myrtilles fourmis

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## #3 2007-02-28 08:26:35

Dross
Member
Registered: 2006-08-24
Posts: 325

### Re: Question by antgaudi

is the number of elements in B that are not in A. You know there are 80 elements in B, and of those, 40 are in A (as
), leaving the remaining 80 - 40 = 40 elements of B not in A, and hence in A[sup]c[/sup].

is the number of elements (out of the entire 200) that are not in B - how many is this?

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## #4 2007-02-28 08:45:36

John E. Franklin
Member
Registered: 2005-08-29
Posts: 3,588

### Re: Question by antgaudi

See tiny depiction.

igloo myrtilles fourmis

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## #5 2007-02-28 09:12:21

mathsyperson
Moderator
Registered: 2005-06-22
Posts: 4,900

### Re: Question by antgaudi

Venn Diagrams are a great way to understand problems like these. If you can work purely with formulas and not get confused then fine, but I find that drawing a picture really helps understanding.

If you get any more questions like these, I recommend drawing a Venn Diagram, looking at the region that you want to find, and working it out from there.

Why did the vector cross the road?
It wanted to be normal.

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