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

*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)=40How can I obtain n(A complement which intersects B)?

What about n(B complement)?Thanks.

Why did the vector cross the road?

It wanted to be normal.

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

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

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?

Bad speling makes me [sic]

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

See tiny depiction.

**igloo** **myrtilles** **fourmis**

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

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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