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# Probability question watch

1. When we have p(A n B) (probability of A intersection B), in what cases can I simply multiply p(A) and p(B) to work this out and when can I not?

In the cases where I can't use multiplication how do I then go about working it out?

I ask because I was doing a question and to work out all the intersection I multiplied the probabilities however most of it was incorrect and I had to manually think about which cards were included in the intersection and divide by 52. (question is attached).
2. Can multiply if they are independent. Otherwise have to work out manually each case unless you are given something extra , like P(A|B) and P(B) which becomes obvious how to find intersection.
3. (Original post by hassassin04)
Can multiply if they are independent. Otherwise have to work out manually each case unless you are given something extra , like P(A|B) and P(B) which becomes obvious how to find intersection.
So in the attached question was it incorrect to multiply together?
4. (Original post by Davelittle)
So in the attached question was it incorrect to multiply together?
I would model this as a Venn diagram (with data rather than probabilities) with 3 overlapping circles
5. (Original post by Davelittle)
So in the attached question was it incorrect to multiply together?
For part ii? Why do you want to multiply them? Just think- which card(s) satisfie all those properties? The probability then should be clear.
6. (Original post by hassassin04)
For part ii? Why do you want to multiply them? Just think- which card(s) satisfie all those properties? The probability then should be clear.
No for part i. when using the formula for the union of 3 things
7. (Original post by Davelittle)
No for part i. when using the formula for the union of 3 things
Same thing. I would just list all cards that satisfy the requirement.

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Updated: December 28, 2014
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