ChatterBank0 min ago
Children
A couple have two children (and they are a normal couple with a 50:50 chance of having a girl or a boy). You are intorduced to one of their children who is a girl. What is the probability that the other child is also a girl? There is a QI claxon waiting for the unwary.....
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Big Dave,
The question does not refer to the probability of giving birth to a girl or a boy which, I agree is as near 50/50 as makes no difference.
The question asks IF you know someone has 2 children & you meet one, what is the probability of the other being the same sex. It's a difficult concept to grasp but if you look at how another-view has laid it out then it becomes clear. As AV says it is irrelevant whether you have met the 1st born or not.
The question does not refer to the probability of giving birth to a girl or a boy which, I agree is as near 50/50 as makes no difference.
The question asks IF you know someone has 2 children & you meet one, what is the probability of the other being the same sex. It's a difficult concept to grasp but if you look at how another-view has laid it out then it becomes clear. As AV says it is irrelevant whether you have met the 1st born or not.
I repeat what I said - the logic is seriously flawed. Look at my first post for the correct answer.
By saying that you have met one of them then you have fixed whether that is the older or younger. Which it is does not matter. You can't then vacillate between the two in the course of the logic.
If you told this at the end of the evening in the pub, you might be able to persuade those present that you had a case.
By saying that you have met one of them then you have fixed whether that is the older or younger. Which it is does not matter. You can't then vacillate between the two in the course of the logic.
If you told this at the end of the evening in the pub, you might be able to persuade those present that you had a case.
For what it is worth, and I do have a maths degree so I know something about probability, the answer of 1/3 is correct.
You can read an explanation here:
http://www.jimworthey.com/puzzle.html
You can read an explanation here:
http://www.jimworthey.com/puzzle.html
I wish I'd never posted the damn Q!!!
For the last time here we go:
You know that there are 2 children.
You know that one of them is a girl.
The question asks about the probability of the other child also being a girl, NOT the probability of giving birth to a girl or boy.
SO: there are 4 possible combinations of having 2 children, B/B, G/G, G/B & B/G
Knowing that one is a girl means you have eliminated B/B from the possibilities, leaving G/G, G/B & B/G as the options.
Therefore, in only one out of those 3 options is the other child a girl, hence the probability of 1/3
For the last time here we go:
You know that there are 2 children.
You know that one of them is a girl.
The question asks about the probability of the other child also being a girl, NOT the probability of giving birth to a girl or boy.
SO: there are 4 possible combinations of having 2 children, B/B, G/G, G/B & B/G
Knowing that one is a girl means you have eliminated B/B from the possibilities, leaving G/G, G/B & B/G as the options.
Therefore, in only one out of those 3 options is the other child a girl, hence the probability of 1/3
