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# Probability watch

1. Hello -having some frustrations with probability! Im nearly crying out of frustrations because somethings I just don't get.

I have been doing this question:

An online readers club has 50 members. Glasses are worn by 15 members, 18 are left handed and 21 are female. There are four females who are left handed, three females who wear glasses and five members who wear glasses and are left handed. only one member wears glasses, is left handed and female.

I have answered all the other questions apart from part d which is driving me insane!

A member is selected at random. Find the probability that the member.

d) wears glasses given that she is left handed and female.

For this I thought you had to use the formula:
P(G|LH and F) = p(G and LH and F)/p(LH and F)

Why is the denominator 4/50 instead of 3/50

I know that the numerator is 1/50. But from the venn digram its 3/50 for left handed AND female....
2. (Original post by christinajane)
Hello -having some frustrations with probability! Im nearly crying out of frustrations because somethings I just don't get.

I have been doing this question:

An online readers club has 50 members. Glasses are worn by 15 members, 18 are left handed and 21 are female. There are four females who are left handed, three females who wear glasses and five members who wear glasses and are left handed. only one member wears glasses, is left handed and female.

I have answered all the other questions apart from part d which is driving me insane!

A member is selected at random. Find the probability that the member.

d) wears glasses given that she is left handed and female.

For this I thought you had to use the formula:
P(G|LH and F) = p(G and LH and F)/p(LH and F)

Why is the denominator 4/50 instead of 3/50

I know that the numerator is 1/50. But from the venn digram its 3/50 for left handed AND female....
Because there are 4 people who are left handed and female
3. (Original post by samb1234)
Because there are 4 people who are left handed and female
But in that section on the venn diagram is says only three - or do you include the 1 which has all three attributes??

Thats why I dont get it....
4. (Original post by christinajane)
But in that section on the venn diagram is says only three - or do you include the 1 which has all three attributes??

Thats why I dont get it....
Yes you do, you need to consider all of the people who are left handed and female regardless of whether they also fit into another category
5. Ahh ok - i get confused with the wording!

I think got confused though with the intersection - so the answer would be 3/50 if they asked for the p(LH intersection F) ??
6. (Original post by christinajane)
But in that section on the venn diagram is says only three - or do you include the 1 which has all three attributes??

Thats why I dont get it....
As an example to show why imagine you have 100 people who play guitar, of whom 50 are girls. Therefore if on your venn diagram you had guitar players as one category, and girls as another you would have 50 in just guitar and 50 un the overlap. If the overlap wasn't counted (essentially what you did before) then the probability of girl |guitar would equal 50/50 =1 which clearly isn't correct, thus showing that you have to consider all the people who satisfy playing guitar, even if they fit into other categories too
7. Thanks for that!

I have to admit I get really confused with the wording and knowing what numbers to use and what not to.

Probability is definitely NOT my strongest subject - I can't always connect the symbols to the wordings and thus the formula you can use.
8. (Original post by christinajane)
Thanks for that!

I have to admit I get really confused with the wording and knowing what numbers to use and what not to.

Probability is definitely NOT my strongest subject - I can't always connect the symbols to the wordings and thus the formula you can use.
Conditional probability is actually fairly easy once you get your head round it - essentially you're told that the person fits into one group, and then asked to find the chance they are in another group as well. It's like if I asked you what % of your form group were also in your maths group - I am telling you that they are in your form group, and you are then calculating the number and hence probability of people in your form who are in your maths class
9. Yeah when you explain it like that it makes a whole lot of sense.

What I was doing was putting numbers into the formula and thinking will the p(F and Left handed) = 3/50 on the venn diagram so it must be that - suppose I have got caught in the trap of that being ok for other questions - but I am sure they have only been dealing with two things where as this question has three...

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