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# Conditional Probability Question - Help

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1. I generally find conditional probability as one of my weak points so any help would be greatly appreciated.

If P(A) = 0.4, P(B) = 0.7, P(A and B) = 0.2

What is P(A'|B)? Also, what would P(A'|B') be?

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2. (Original post by nemanuel96)
I generally find conditional probability as one of my weak points so any help would be greatly appreciated.

If P(A) = 0.4, P(B) = 0.7, P(A and B) = 0.2

What is P(A'|B)? Also, what would P(A'|B') be?

Posted from TSR Mobile
Drawing a Venn Diagram really helps for these questions. So draw one with the three sections containing 0.4, 0.2 and 0.7.

P(A'|B) : This means the probability of "not A" given B. So you can ignore everything that is not in the B circle.

Then how much of the B circle is also "not A"?

Post your ideas / working if you get stuck.
3. (Original post by notnek)
Drawing a Venn Diagram really helps for these questions. So draw one with the three sections containing 0.4, 0.2 and 0.7.

P(A'|B) : This means the probability of "not A" given B. So you can ignore everything that is not in the B circle.

Then how much of the B circle is also "not A"?

Post your ideas / working if you get stuck.
be careful not to write these numbers straight onto your diagram... they add up to more than 1 ....
4. Sorry guys, I solved this about a minute after I posted this haha. Thanks for your help though, much appreciated!

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5. (Original post by the bear)
be careful not to write these numbers straight onto your diagram... they add up to more than 1 ....
Wow... I can't believe I typed that!

Thanks.
6. Perhaps you could help me on another question that I'm stuck on, but I'm not entirely sure whether it's an error on my behalf or theirs:

Given that P(A) = 0.8, P(A|B) = 0.8, P(A and B) = 0.5.

Find P(B|A).

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7. (Original post by nemanuel96)
Perhaps you could help me on another question that I'm stuck on, but I'm not entirely sure whether it's an error on my behalf or theirs
We won't be able to tell whether it's an error on your behalf or theirs unless you post your working.
8. (Original post by Zacken)
We won't be able to tell whether it's an error on your behalf or theirs unless you post your working.
I done P(B and A)/P(A) = 0.625. They managed to get 37/40 or 0.925. Not entirely sure how to get that answer, tried pretty much everything.

Thank you in advance.

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