A simple logic problem

[Illini]

We are not talking about looking at odds once you have the first child, we
are talking about looking at families that have already had two kids.

There is a huge difference.

You are talking like the fact that the first child is a boy means something.

It doesn't. We know NOTHING about the second child. It could be a boy, it could just as easily be a girl.

Any amount of money you want to bet.

 

[Illini]

1. BB
2. GG
3. GB
4. BG

We know it's not going to be 2 or 3.

It can only be 1 or 4. 50%. I don't know WTF you people are smoking to say otherwise.

 

[viking611]

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[JaBrady]

1. BB
2. GG
3. GB
4. BG

We know it's not going to be 2 or 3.

It can only be 1 or 4. 50%. I don't know WTF you people are smoking to say otherwise.

why can't it be 3? you dont know the order of the birth

 

[festeringZit]

1. BB
2. GG
3. GB
4. BG

We know it's not going to be 2 or 3.

It can only be 1 or 4. 50%. I don't know WTF you people are smoking to say otherwise.

Here is the original problem yet again (sigh).

A man tells you he has two children. He then starts talking about his son. He does not tell you whether the son is the oldest child or the youngest child. What is the probability that his other child is a girl?

This does not rule out GB, the possibilities are:

BB 1/3
BG 1/3
GB 1/3

 

[Illini]

why can't it be 3? you dont know the order of the birth

hno: I'm done explaining.

 

[festeringZit]

You are talking like the fact that the first child is a boy means something.

It doesn't. We know NOTHING about the second child. It could be a boy, it could just as easily be a girl.

Any amount of money you want to bet.

Illini, we have a history on here, and I consider you a friend. I don't
want to take your money.

Again, this is a old *simple* problem in probability theory, first published
in 1959 in Scientific American, p.180. There is no disputing the answer,
it is 2/3.

 

[Geoff101]

I think we can all agree that laying 3/2 in a survey that someone's second child is a girl would leave Zit homeless.

But that's what he is professing....that what he says the odds are...cant we just TRY it???:nohead:

 

[Illini]

Here is the original problem yet again (sigh).

A man tells you he has two children. He then starts talking about his son. He does not tell you whether the son is the oldest child or the youngest child. What is the probability that his other child is a girl?

This does not rule out GB, the possibilities are:

BB 1/3
BG 1/3
GB 1/3

You have two possibilities left. BB and BG. That's it. One is a boy. The other child could be a girl or it could be a boy. We don't know.

 

[Illini]

Illini, we have a history on here, and I consider you a friend. I don't
want to take your money.

Again, this is a old *simple* problem in probability theory, first published
in 1959 in Scientific American, p.180. There is no disputing the answer,
it is 2/3.

If you talking about all couples with 2 and only 2 children, and comparing this probability in relation to all other 2 children couples, then yeah, you can draw that conclusion.

But you aren't. You are talking about a man that has a son and 1 other child. You don't know anything about that other child. It might be a girl (50%) and it might be a boy (50%).

 

[JaBrady]

not perfect but I thought of this scene in a movie related to statistics and variable change (skip to 3:45 mark)

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[festeringZit]

If you talking about all couples with 2 and only 2 children, and comparing this probability in relation to all other 2 children couples, then yeah, you can draw that conclusion.

But you aren't. You are talking about a man that has a son and 1 other child. You don't know anything about that other child. It might be a girl (50%) and it might be a boy (50%).

Given that you find a family with two kids and they aren't both girls,
you are twice as likely to find one of each, than you are of having
both boys.

Think about that for a while. Once the truth of that statement hits you, you'll understand the 2/3 answer.

 

[fhmesq44]

Do this, it will be unscientific but representative. Illini, Geoff, Zit, whoever else, think of everyone you know that has two children who can make a statement using the term "son", then figure out how many of them have a daughter and how many have two boys and post those numbers in this thread.

Like I said, unscientific, but may illustrate why the answer is 2/3. I will do the same.

 

[JakeT.]

So what's the correct answer?

 

[Geoff101]

but that wasnt the conundrum...there arent 2 variables, just ONE....there is only a question as to what the sex of ONE of the children is...not both of them

 

[fhmesq44]

but that wasnt the conundrum...there arent 2 variables, just ONE....there is only a question as to what the sex of ONE of the children is...not both of them

Dude, that is exactly what you said you wanted to drive to Austin to test out. That is the question.

Man: I have two children and one is a boy.

Geoff: What is your other child?

 

[GoForBroke]

so what's the correct answer?

2/3.

 

[timotinbowen]

Allright, it's good to see that there is active debate taking place. The correct answer is 2/3. Geoff, if we knew the FIRST child was male, then there is a 50% chance that the second child is male. But if we just know that there is at least one male in the set of two children, then there is a 2/3 probability that the other is female. We are NOT being told the gender of a specific child, and that is what is important. As FesZit already explained, Bayes' Theorem could be used to prove this, but you can also get to the answer using only logical thinking.

The examples people have used about having 20 kids and knowing one is a boy does not fit well, because then we are only ruling out one subset from 2^20 combinations (20 girls), which has very little statistical significance in determining the M:F ratio in the family.

Note that there is a very big difference between GB and BG.

 

[festeringZit]

Do this, it will be unscientific but representative. Illini, Geoff, Zit, whoever else, think of everyone you know that has two children who can make a statement using the term "son", then figure out how many of them have a daughter and how many have two boys and post those numbers in this thread.

Like I said, unscientific, but may illustrate why the answer is 2/3. I will do the same.

Marilyn Vos Savant posed this same problem in one of her columns,
and they ended up doing a survey and got 18,000 responses. She
worded it to find the probability given no girls, that they both are boys.

"Finally vos Savant started a survey, calling on women readers with exactly two children and at least one boy to tell her the sex of both children. With almost eighteen thousand responses, the results showed 35.9% (a little over 1 in 3) with two boys."

http://www.answers.com/topic/marilyn-vos-savant

That should finally put all the BS on here to rest.

 

[Teddykgb]

I'm a High School drop out and this one is a no brainer even for me......it's 50% YOU FVCKING QUEENS!!!!!!! ZIT you are an idiot!!!!!!!!!!