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# S1 Probability Watch

1. Stuck on part b.
2. (Original post by zed963)

Stuck on part b.
I think we had this question come up a couple of days ago

Some people like to tackle these by Venn diagrams; I personally prefer the algebraic approach, but it's up to you.

To start with, use some letters to represent the events you're told about. Can you write down the information you're given in terms of symbols, and what is the probability that they are asking for in part (b) in terms of those symbols?
3. (Original post by davros)
I think we had this question come up a couple of days ago

Some people like to tackle these by Venn diagrams; I personally prefer the algebraic approach, but it's up to you.

To start with, use some letters to represent the events you're told about. Can you write down the information you're given in terms of symbols, and what is the probability that they are asking for in part (b) in terms of those symbols?
p(E'nB')
4. (Original post by zed963)
p(E'nB')
Great - that looks like the same sort of notation that I would have chosen

The next step is to write down all the info they give you e.g. P(E) = ? and also any useful formulae you remember e.g. P(EnB) = P(E|B) x P(B) etc and see if you can manipulate the info they give you to get the required probability,
5. (Original post by davros)
Great - that looks like the same sort of notation that I would have chosen

The next step is to write down all the info they give you e.g. P(E) = ? and also any useful formulae you remember e.g. P(EnB) = P(E|B) x P(B) etc and see if you can manipulate the info they give you to get the required probability,
p(E)=0.4

P(EnB)=9/25

I'm not sure what P(ElB) is

P(B)=2/3
6. (Original post by zed963)
p(E)=0.4

P(EnB)=9/25

I'm not sure what P(ElB) is

P(B)=2/3
Unless I've misread the question, I think P(E|B) is what they've given you - it's the probability that someone takes exercise given that they eat breakfast.
7. (Original post by davros)
Unless I've misread the question, I think P(E|B) is what they've given you - it's the probability that someone takes exercise given that they eat breakfast.
so 9/25?
8. (Original post by zed963)
p(E)=0.4

P(EnB)=9/25

I'm not sure what P(ElB) is

P(B)=2/3
Isn't P(E|B)=9/25 and P(EnB) what you worked out for part A?
I might be wrong
9. (Original post by Super199)
Isn't P(E|B)=9/25 and P(EnB) what you worked out for part A?
I might be wrong
I hate probability so
10. (Original post by zed963)
I hate probability so
I just went looking for the other thread on this problem and it looked like you were the one who started it! You really should stick to one thread

Anyway,

they want you to calculate P(E'nB').

Now, E'nB' is the complementary event to EUB (check on a Venn diagram if you're not sure why!) so P(E'nB') = 1 - P(EuB).

Can you see how to proceed from here?
11. (Original post by davros)
I just went looking for the other thread on this problem and it looked like you were the one who started it! You really should stick to one thread

Anyway,

they want you to calculate P(E'nB').

Now, E'nB' is the complementary event to EUB (check on a Venn diagram if you're not sure why!) so P(E'nB') = 1 - P(EuB).

Can you see how to proceed from here?
9/25*2/3=6/25

1-6/25=19/25 ?
12. (Original post by zed963)
9/25*2/3=6/25

1-6/25=19/25 ?
I'm not sure what you're working out there!

You want to get to P(E'nB'). To do this you need P(EuB). How do you calculate P(EuB) when you know P(EnB)?
13. (Original post by davros)
I'm not sure what you're working out there!

You want to get to P(E'nB'). To do this you need P(EuB). How do you calculate P(EuB) when you know P(EnB)?
Oh I didn't understand the question and that's probably why I struggle so much.

P(EuB)=P(E)+P(B)-P(EnB)

P(EuB)=2/5 + 2/3 - 18/75
14. (Original post by zed963)
Oh I didn't understand the question and that's probably why I struggle so much.

P(EuB)=P(E)+P(B)-P(EnB)

P(EuB)=2/5 + 2/3 - 18/75
P(E'nB')=13/75
15. P(E'nB') is equivalent to P(E U B)'
16. (Original post by the bear)
P(E'nB') is equivalent to P(E U B)'
I'm crap with probability is that the outside bit of a venn diagram?
17. (Original post by Super199)
I'm crap with probability is that the outside bit of a venn diagram?
yes, yes it is

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