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# Stuck on Probability Question watch

1. We have events A and B

P(A)= 1/5 and thus P(A')= 4/5

P(B¦A)= 1/4

P(B¦A')= 3/8

How do I find P(B)?

ps. Sorry for the weird notation on the B given that it's A/A'... Couldn't find the vertical line on my keyboard!
2. (Original post by lalilou)
We have events A and B

P(A)= 1/5 and thus P(A')= 4/5

P(B¦A)= 1/4

P(B¦A')= 3/8

How do I find P(B)?

ps. Sorry for the weird notation on the B given that it's A/A'... Couldn't find the vertical line on my keyboard!
| is to the left of Z on your keyboard, probably.

Using the standard formula for conditional probablity, apply it to P(B|A), and see what you can work out. Similarly for the other one.

For the last step, if you can't think of it, look in the spoiler

Spoiler:
Show

P(B) = P(BnA) + P(BnA')

3. I think this is the way to do it:
P(B) = [P(B) and P(A) ] divided by the P(A)
4. (Original post by ghostwalker)
| is to the left of Z on your keyboard, probably.

Using the standard formula for conditional probablity, apply it to P(B|A), and see what you can work out. Similarly for the other one.

For the last step, if you can't think of it, look in the spoiler

Spoiler:
Show

P(B) = P(BnA) + P(BnA')

Thank you!! Ironically, just as I posted the thread, I looked down at my diagram and realised how simple it actually was. Now I feel silly for posting!

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