## Welcome to BrainDen.com - Brain Teasers Forum

Welcome to BrainDen.com - Brain Teasers Forum. Like most online communities you must register to post in our community, but don't worry this is a simple free process. To be a part of BrainDen Forums you may create a new account or sign in if you already have an account. As a member you could start new topics, reply to others, subscribe to topics/forums to get automatic updates, get your own profile and make new friends. Of course, you can also enjoy our collection of amazing optical illusions and cool math games. If you like our site, you may support us by simply clicking Google "+1" or Facebook "Like" buttons at the top. If you have a website, we would appreciate a little link to BrainDen. Thanks and enjoy the Den :-) |

# One Girl - One Boy

### #341

Posted 17 July 2012 - 03:54 PM

The OP says "One," so it appropriately asks about the "other."

But be careful not to identify "One" with a

**specific**one.

That leads to 1/2 unambiguously, but the OP does not support that interpretation

*Vidi vici veni.*

### #342

Posted 18 July 2012 - 04:04 PM

I believe the mere presence of "other" in the OP implies that our given information pertains specifically to one individual (not two collectively) and the question asks us to infer information about the "other" individual.That leads to 1/2 unambiguously, but the OP does not support that interpretation

At its heart, the 1/3-ist interpretation is that the given information pertains to the joint distribution of two individuals, and the solution is an inference that can be made about the conditional joint distribution (i.e. the fact that in the FF case, we may conveniently neglect the issue of selecting an "other" and conclude "female" since both siblings are female).

But perhaps this speaks to the point of many posts in this forum, which is that the problem is poorly stated. The answer depends on what specifically is meant by "two kids; one girl"--how that information is obtained.

Thanks to all for the discussion, at any rate.

**Edited by syonidv, 18 July 2012 - 04:07 PM.**

### #343

Posted 19 July 2012 - 04:48 PM

I believe the mere presence of "other" in the OP implies that our given information pertains specifically to one individual (not two collectively) and the question asks us to infer information about the "other" individual.

At its heart, the 1/3-ist interpretation is that the given information pertains to the joint distribution of two individuals, and the solution is an inference that can be made about the conditional joint distribution (i.e. the fact that in the FF case, we may conveniently neglect the issue of selecting an "other" and conclude "female" since both siblings are female).

But perhaps this speaks to the point of many posts in this forum, which is that the problem is poorly stated. The answer depends on what specifically is meant by "two kids; one girl"--how that information is obtained.

Thanks to all for the discussion, at any rate.

Well said. If the OP had said "It's not the case that they are both boys. What is the probability that they are both girls?" this forum would have been much shorter.

Thanks to you, for posting your thoughts clearly and cogently.

*Vidi vici veni.*

### #344

Posted 14 April 2013 - 07:16 PM

### #345

Posted 14 April 2013 - 09:09 PM

At first, I thought this 35 page long argument was ridiculous, but after reading bona/syon's posts above, I can understand some of the trouble surrounding the ambiguity of the question. This is the way I see it:

There are four possibilities for boy/girl combinations, as mentioned several times by now (I imagine):

BB

BG

GB

GG

Note that this categorization* does* take into account the order of the children. We are shown that one of the children is a girl, which of course eliminates the first combination.This is the point where people jump to the answer of 1/3, since there only seem to be three combinations remaining. However, as I stated before, order does matter if you look at the problem this way. Therefore, there are still four possibilities:

**G1**B2

**G2**B1

**G1**G2

**G2**G1

Where the number indicates order. The **girl** we see can be either the elder (G1) or younger (G2) sister, so we must look at both possibilities equally. Thus, the probability stands at 1/2, regardless of the order of birth.

Another way to look at this is to completely disregard order in all of the cases. The resulting combinations are:

BB

**G**B (same as BG!)

**G**G

When BB is eliminated, the probability of GG remains at 1/2.

**Edited by ParaLogic, 14 April 2013 - 09:10 PM.**

### #346

Posted 14 April 2013 - 09:17 PM

The probability that they are both girls is 1/2. We know that one child is a girl. This does not affect the probability of what the other child is. If you are equally likely to have a boy or a girl, having another child does not make any change to that. In fact, there are only 3 possible combinations of gender, two boys, two girls, and one of each. Since two boys are not possible, there is a 1/2 probability of the other child being a girl.

- If OP is taken to identify "one" of the two, and to say that "one" is a girl, the probability is indeed 1/2 that the "other" is, too.

- If OP is taken to say it is not the case that they are both boys, then one can deduce a probability of 1/3, although it is not necessary to conclude 1/3 since our fact-reporter's algorithm has not been disclosed.

- It is incorrect to say there are 3 combinations and one is removed so p=1/2. That reasoning ignores the equal-likelihood requirement: I buy a lottery ticket, and there are only two possibilities: winning and losing. Therefore my chances of winning are ...

- Before adding to this thread, read through some of the posts first.

- Additional, thoughtful comments are always appropriate.

*Vidi vici veni.*

### #347

Posted 20 September 2013 - 12:49 PM

I think discussion so far might have not satisfied many learned friends. May be my explanation would clarify some doubts.

OP says that the couple has two kids, so obviously one will be younger and other elder.

Say couple sends elder kid to attend a function. Now what is the probability of this kid being a girl, if one kid is definitely a girl...?

I think you got it it now....!

Say younger is a girl, then there are two possibilities: elder may be a boy or a girl.

As bonanova stated correctly, OP does not specifies which kid is a girl. so younger kid could be a boy, so the third possibility is that elder could be a girl; here fourth possibility of elder being a boy, when younger is a boy, is ruled out because OP says that one kid is definitely a girl.

Now out of above three possibilities, there is only one possibility of both kids being girls.

So the answer is 1/3.

### #348

Posted 05 November 2013 - 03:42 AM

Shouldn't it be a 1/2 as one child is a girl and that another child can be a boy or a girl.

### #349

Posted 22 November 2013 - 05:59 PM

Not necessarily. What you say is correct: there are only two possibilities. But are they equally likely? Try thinking in terms of "they are not both boys." It's logically equivalent. Do you still get 1/2?Shouldn't it be a 1/2 as one child is a girl and that another child can be a boy or a girl.

*Vidi vici veni.*

#### 0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users