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s1 maths probability question help

The events A and B are such that P(A) =2/5 , P(B) =1/2 and P(A given B') =4/5 .
(a) Find
(i) P(A n B ')
(ii) P(A n B),
(iii) P(A u B),
I've solved the first one but cant do the second one.
(edited 7 years ago)
Might just be an issue for me but I can't see P(A), P(B) and it just displays 'P(AB ) = .'
Reply 2
Avatar for han009
han009
i think the second one just means probability of A and B so thats 0.4 multiplied by (1-05) = 0.2 ?
and the third one is A or B or both, so I think thats 0.4 (0.5) + 0.4 + 0.5 = 1.1 - this doesnt make sense so i dont know pls let us know the answer
Reply 3
Avatar for towelrack
towelrack
OP
Original post by han009
i think the second one just means probability of A and B so thats 0.4 multiplied by (1-05) = 0.2 ?
and the third one is A or B or both, so I think thats 0.4 (0.5) + 0.4 + 0.5 = 1.1 - this doesnt make sense so i dont know pls let us know the answer


the mark scheme does the follwoing;
P(A n B) = P(A) – P(A n B')
but i ave no idea why...
Original post by towelrack
The events A and B are such that P(A) =2/5 , P(B) =1/2 and P(A given B' ) =4/5 .
(a) Find
(i) P(A n B ' )
(ii) P(A n B),
(iii) P(A u B),
I've solved the first one but cant do the second one.


P(A∣B′)=P(A∩B′)P(B′)\displaystyle P(A\vert B' )=\frac{P(A \cap B' )}{P(B' )}

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