Labrador99
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#1
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Hi

This is probably a really simple question, but I just wanted to check I'm doing this right.

The question gives:
P(A)= 0.5, P(AuB)= 0.6 and P(AnB)=0.2

Part a asks to find P(B), which I got using
P(A) + P(B) - P(AnB) = P(AuB), to give P(B)= 0.3 (which is the correct answer)

Part b is to find P(A'nB). I have got 0.1 for this (which is the right answer, but I'm not convinced my working is right). Can someone please confirm whether I've done the right thing?
P(A'nB) is when it's not A, but it is B, so (since there are occasions where you can get A and B, and also when it's neither), I have done:
P(A'nB)= P(B) - P(AnB)
= 0.3-0.2
= 0.1

This gives the right answer, but can someone just check whether my logic is right (and confirm it's not just a lucky mistake!)?

Thanks
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ghostwalker
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#2
Report 2 years ago
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(Original post by Labrador99)
Hi

This is probably a really simple question, but I just wanted to check I'm doing this right.

The question gives:
P(A)= 0.5, P(AuB)= 0.6 and P(AnB)=0.2

Part a asks to find P(B), which I got using
P(A) + P(B) - P(AnB) = P(AuB), to give P(B)= 0.3 (which is the correct answer)

Part b is to find P(A'nB). I have got 0.1 for this (which is the right answer, but I'm not convinced my working is right). Can someone please confirm whether I've done the right thing?
P(A'nB) is when it's not A, but it is B, so (since there are occasions where you can get A and B, and also when it's neither), I have done:
P(A'nB)= P(B) - P(AnB)
= 0.3-0.2
= 0.1

This gives the right answer, but can someone just check whether my logic is right (and confirm it's not just a lucky mistake!)?

Thanks
Your working is fine.

Try sketching the relevant areas on a Venn diagram.
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Labrador99
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Report Thread starter 2 years ago
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(Original post by ghostwalker)
Your working is fine.

Try sketching the relevant areas on a Venn diagram.
Thank you

That's really helpful advice - makes it much easier!
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Labrador99
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#4
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Didn't want to make a new thread on the same topic, but can anyone help me with this question on conditional probability? I don't have any attempt to show, because I'm not really sure where to start with it.

When Alan and Elizabeth play chess the probability that Alan will win the first game is p. If he wins the first game then his chances of winning the next game are doubled. However, if Elizabeth wins the first game then she has the probability p of winning the next game. Find p, given that the probability that Alan wins one and loses one of the first two games is five ninths.

Any help much appreciated
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ghostwalker
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(Original post by Labrador99)
Didn't want to make a new thread on the same topic, but can anyone help me with this question on conditional probability? I don't have any attempt to show, because I'm not really sure where to start with it.

When Alan and Elizabeth play chess the probability that Alan will win the first game is p. If he wins the first game then his chances of winning the next game are doubled. However, if Elizabeth wins the first game then she has the probability p of winning the next game. Find p, given that the probability that Alan wins one and loses one of the first two games is five ninths.

Any help much appreciated
A probability tree would be one way to start.
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