S1 Probability

zed963

Stuck on part b.

Reply 1

davros

Original post by zed963

Stuck on part b.

I think we had this question come up a couple of days ago :smile:

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?

Reply 2

zed963

Original post by davros

I think we had this question come up a couple of days ago :smile:

p(E'nB')

Reply 3

davros

Original post by zed963

p(E'nB')

Great - that looks like the same sort of notation that I would have chosen :smile:

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,

Reply 4

zed963

Original post by davros

Great - that looks like the same sort of notation that I would have chosen :smile:

p(E)=0.4

P(EnB)=9/25

I'm not sure what P(ElB) is

P(B)=2/3

Reply 5

davros

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.

Reply 6

zed963

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?

Reply 7

Super199

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 :redface:

Reply 8

zed963

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 :redface:

I hate probability so :angry:

Reply 9

davros

Original post by zed963

I hate probability so :angry:

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 :biggrin:

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?

Reply 10

zed963

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 :biggrin:

9/25*2/3=6/25

1-6/25=19/25 ?

Reply 11

davros

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 have already calculated P(EnB).

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)?

Reply 12

zed963

Original post by davros

I'm not sure what you're working out there!

You have already calculated P(EnB).

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)?