[Closed] The boy-girl puzzle

[i]Two kids, you know the sex of one. How many does that leave whose sex you don't know?[/i]

2

What is the sex of the eldest child?... don't know
What is the sex of the youngest child?... don't know

3

What is the sex of the child with brown hair? Don't know.

Y'know - I can't help thinking that having a huge thread that continually uses the words "child" and "sex" might somehow attract the wrong sort of people using Google.

lol

The kind that posts pics of Stephanie from Lazy Town?

There's a horse race with 6 horses:

Horse A will either win or lose - 50:50
Horse B will either win or lose - 50:50
Horse C will either win or lose - 50:50
Horse D will either win or lose - 50:50
Horse E will either win or lose - 50:50
Horse F will either win or lose - 50:50

(Won't help the argument at all, but I wanted to add the word "horse" to the search mix.)

Well done mboy your question has sufficient info to work

I only threw the extra dimension in to add a touch of confusion to the situation for those that don't understand, seems to have worked! 😉

Andy... but it doesn't leave a single unknown, you are just making that assumption.

The question put by mboy, gives you three unknowns, but that third one is irrelevant. The original difference, birth order, is still there even without the race issue. You could just as well do it where one child has ginger and one child has blonde.

Yup, exactly, the 3rd unknown I created is totally irrelevant, I threw it in there to add confusion as stated above. It's only the same as the Cat/Dog situation created slightly earlier on in this thread though.

Out of the 4 combinations you have all ruled out boy/boy.

That leaves

girl/boy
boy/girl
girl/girl

show me how to figure out the probability that the first one is a boy....

We might get ten pages after all!

Why do you want to know the probability that the first one is a boy?

Just want to know.

No answer to that one?

Boy Girl is, in this case, the same as girl boy. She has one girl, so the question may as well be "what are the chances that a woman's first and only child is a boy?"

easy, 50/50 😆

...what are the chances of twins?

can't be bothered to check but sometime back pointed out that the "rules" didn't exclude the lady from being a liar - the result assumes she is telling the truth

Smee: from a population equally divided into GB, BG, GG - the chances of the "first" one being a boy are 1:2 aka 1 in 3 aka 33%.
Not sure how this is relevant to the question tho, or your cat's arse and the sun.

Beveled Edge: no, you are pre-selecting. You don't have that information at the start of the question. That is equivalent to saying that in the Monty Hall problem the host says "pick any door from these three, except that one."

GrahamS - I didn't ask anything about a population being equally divided between the three options. I asked what was the probability of the first child being a boy. Any answer other than 50:50 is obviously incorrect.

You didn't state that, but it is vital information that we need to calculate the probability.

If we are still talking about the population in our problem then we have already established that it is equally split, so the answer is one third.

I have literally no idea how you can look at those 3 options and decide it is 50%

What does that mean the probabilty is for the second one being a boy???

Didn't need to state it. The answer to the probability of the first child being a boy is only ever going to be 50% somehow you managed to come up with 33%. And you're telling me that i'm wrong.

Probability of any other child being a boy is 50%

Right I think you have finally talked yourself into a corner. You say 50% so that means...

First child is a boy = 50%
Second child is a boy = 50%
Both are girls = 0%

Do you notice anything there?

If I am misinterpreting then please fill in your "correct" values.

Bear in mind that there is also a 50% chance that the first child is a girl - same as with any other child.

So your breakdown of the probabilities is....?

....is?

Well it seems to me it all hinges on whether boy/girl is the same as girl/boy. Under current discrimination laws we must not discriminate on sex or age and therefore we must assume boy/girl is the same as girl boy. Thus 50% seems to be the politically correct answer.

first child is a boy = 50%
second one is a boy = 50%
Both are girls = 50%

Because
first child is a boy = 50% it is a girl = 50% = 100%
second one is a boy = 50% it is a girl = 50% = 100%
Both are girls = 50% both are not girls = 50 % (as above)- there can be only two outcomes here two girls or boy /girl ORDER in this example is NOT a factor -

Don't know or care whether any of this is correct just going for the 10 pages and will argue with you if it help GrahamS

Can I just add that I think both logic and maths answers are correct it is just like arguing over imperial verus metric measurements you get different answers but WITHIN their own system (following their own axioms) it is correct or true (which is not the same as TRUTH) so does it really matter?
Also think this is just a paradox as both (conflicting ) answers are correct
Surely given you enough options/ammunition now to continue the thread

He's buggered off, makes a mess of trying to be clever again then tries to change what he meant in the question.

I went out on my bike. Got a puncture 20 odd miles from anywhere then ripped the valve off the spare tube.. Disco slippers are not great for walking in

What's the odds of that!

Lets play 10 pages roulette. Is it this post that'll get to 10 pages?

10 pages of you looking like an idiot? Works for me.

Blows kisses at nickc.

The only idiocy that is going on is people not realising that both solutions are correct.....

"boy girl puzzle" in google only gets STW to 2nd spot, more posts required 😉

Smee, play the devils advocate, try seeing it from the other side of the argument for a minute.

There is information you are assuming currently that does not actually exist, and there is also information that we do know that you are trying to ignore.

Smee - Member

The only idiocy that is going on is people not realising that both solutions are correct.....

Your solution is only correct if the children do not yet exist. Then of course the probability of the first child being a girl is 50%, as it is the second child. With the information given in the question though, we know both children already exist, and at least one is a girl. This of course removes the 25% probability option of Boy/Boy existing, and therefore we have the 3 options stated previously remaining, each at 33.33% as probabilities always have to add up to 100%.

Boy/Girl IS NOT the same as Girl/Boy. Cannot be, never ever will be. If you have 2 children, one is a 12 year old girl and the other a 9 year old boy, is that the same as having a 12 year old boy and a 9 year old girl?

Not reached 10 pages yet but Smee is now claiming both answers can be right, they can but not for the same question.

Who would have thought that the such a long thread would involve Glupton/Smee arguing with people?

What's the odds of that?

Blimey so close to 10 pages all is quiet.

Scotland's currently looking at methods of improving adult literacy and numeracy, as according to UNESCO 23% of Scots have trouble with these. It looks like this help may have come too late for some.

I'm not reading all that!!

How many chuffin' posts??

When will it get to 10...

When you post 🙂

Mboy please show me in the question where it specifically asks us to determine the chances of the woman having had the children in a specific order.

avdave2 - Member

Mboy please show me in the question where it specifically asks us to determine the chances of the woman having had the children in a specific order.

It doesn't!

My point entirely...

What's your point?

[i]Mboy please show me in the question where it specifically asks us to determine the chances of the woman having had the children in a specific order. [/i]

Exactly why it's 66% something I couldn't understand at first.

Blimey, I can't believe I've been away for the weekend and not only is this thread is still ticking along, but Smee seems to be gaining supporters like some kind of strange cult leader.

A few of the more interesting replies:

[b]avdave2:[/b] Under current discrimination laws... 50% seems to be the politically correct answer.

Nah you'd still be discriminating on sex. You'd be safer saying you were 100% that they were a person - though obviously ethereal lifeforms should also have equal rights. 🙂

[b]Junkyard[/b]: Don't know or care whether any of this is correct just going for the 10 pages and will argue with you if it help GrahamS

I think Smee is doing enough trolling for everyone on this thread!

[b]avdave2:[/b]please show me in the question where it specifically asks us to determine the chances of the woman having had the children in a specific order.

It doesn't. But it is the reason why there are twice as many "boy and girl" combinations as there are "girl and girl". In the initial population, 25% will be "two boys", 50% will be "girl and boy", 25% will be "two girls".

[b]Smee:[/b]The only idiocy that is going on is people not realising that both solutions are correct.....

For the question that I gave there is only one correct answer and that is 66.666..%

That is not really debatable. You can go and physically count the results on that spreadsheet and it comes to 66%. If you don't trust the spreadsheet you can replicate the experiment yourself using coins and it is 66%. You can even do what [url= http://en.wikipedia.org/wiki/Marilyn_vos_Savant ]Marilyn vos Savant[/url] did when she ran a poll of her readers and, with 18,000 replies, it came out almost exactly at 66%.

You might not like or understand the reasoning, but the result is not debatable.

I'm sure 50% is the "correct solution" to many interesting questions, but just not this one.

I notice that you still haven't replied what you think the correct values are for your earlier post:

Out of the 4 combinations you have all ruled out boy/boy.

That leaves

girl/boy
boy/girl
girl/girl

show me how to figure out the probability that the first one is a boy...

I said 33% and explained why. I think you said 50% for each of them, but failed to explain why your probabilities added up to 150%.

Is this post still going?!

Yes.

Carry on then...

It's Smee's ball.

I think he's took it home.

Play cancelled due to snow?

Nah! He's just gone home.

You got much at Wylam often do they because it's location?

Who cares?

Me that's why I asked.

You got much at Wylam often do they because it's location?

Yes snow some often do location is 😛

Actually I'm working up in Edinburgh this week. I think we actually got a fair bit in Wylam last night, but yeah it is pretty low lying so we're not likely to be too badly affected. (Not sure if I'll be taking the A68 home though - I suspect going through the Pennines would be fun!)

I would have thought STW would have gone home by now as well. Any people still in the office? Anyone?

i got to page 6 and then realised the answer 0.666666 is being explained very badly

g/b and b/g ARE the same or interchangeable. i would've thought that'd be obvious to everyone on here. it's the MIXED option - it dosen't matter which way round they are.

that leaves 3 options:

mixed
girl/girl
boy/boy

she's already told you one's a girl so that eliminates boy/boy.

so that leaves 2/3 options and here's the bottom line....

2/3 = 0.6 recurring!!

And that's your answer folks, none of this 2:1 bullshirt.

[i]Yes snow some often do location is[/i]

lol! God damn it my dyslexia really shines through when I'm on nights.

WOW! Bomberman has some how made it even more confusing.

i got to page 6 and then realised the answer 0.666666 is being explained very badly

g/b and b/g ARE the same or interchangeable. i would've thought that'd be obvious to everyone on here. it's the MIXED option - it dosen't matter which way round they are.

that leaves 3 options:

mixed
girl/girl
boy/boy

she's already told you one's a girl so that eliminates boy/boy.

so that leaves 2/3 options and here's the bottom line....

2/3 = 0.6 recurring!!

And that's your answer folks, none of this 2:1 bullshirt.

Right answer, wrong reasoning 🙂

Yep - what mike said.

the question asks what are the chances of her having a mixed pair.

mixed
girl girl
boy boy (eliminated)

two options left therefore chances of mixed = 1/2 = 0.5

50 percent. 50/50.

a one in two chance, since she eliminated the boy/boy option.

the question asks what are the chances of her having a mixed pair.

mixed
girl girl
boy boy (eliminated)

two options left therefore chances of mixed = 1/2 = 0.5

50 percent. 50/50.

Nope.

yep

There's a 2 in 3 chance, so 66%

no there isn't a 2 in 3 chance, because there were only 3 options to start off with and one has been knocked out so it's now down to a one in two chance

Dear God. How can you have read up to page 6 and come to that conclusion?

Just because there are two options DOES NOT MEAN that they are 50/50.

If there are 4999 girls and 1 boy in a room and I pick a child at random then the odds ARE NOT 50/50 that I pick the boy - despite the fact that yes, there are only two outcomes and yes, when the children were born it was 50/50 whether they were boy or girl.

For the sake of some people on here's sanity, I'd just like to don my "Mark & Lard" voice and ask for a STOP..................

Carry on..............

😉

youre muddling the issue grahamS
the question asks "what are the odds that she has one boy and one girl?"

you've got only 2 options left, one of which is MIXED

if someone puts a red ball and a blue ball in a bag, the chances of pulling out either are 50/50. Its simple and i dont know why youre adding in all the other stuff

[i]if someone puts a red ball and a blue ball in a bag, the chances of pulling out either are 50/50. Its simple and i dont know why youre adding in all the other stuff [/i]

It's not the same.

it is the same. two options left girl/girl, girl/boy. write them down on a peice of paper, put them in a bag, pick one out. what are the chances of you picking the mixed pair? 50/50.

grahamS the chances of you picking the boy in the room is 1/5000 or 0.02%

if someone puts a red ball and a blue ball in a bag, the chances of pulling out either are 50/50.

Not if there are more red balls than blue balls it isn't!

And that is exactly what is happening here.
There are twice as many "mixed" as there are "two girl".

If you started with 100 of these families then 25 of them would be two boys, 50 would be mixed and 25 would be two girls. If you then eliminate the 25 with two boys then you are 50 mixed and 25 with two girls. Pick one of them at random and you have a 2:1 (aka 66% aka 2/3) chance that it is a mixed family.

bomberman, there are [b]three[/b] options left.

If you write "boy/girl", "girl/boy" and "girl/girl" on three bits of paper and pick out one randomly, what is the chance of you getting one of the two with a boy on it?

no there arent twice as many mixed you CRETIN. girl/boy is the same as boy/girl. if someone asks you how many kids youve got it dosent matter wether you say "a girl and a boy" or "a boy and a girl" there are two ways of saying it but they both mean the same thing YOU PENARSE

it is the same. two options left girl/girl, girl/boy. write them down on a peice of paper, put them in a bag, pick one out. what are the chances of you picking the mixed pair? 50/50.

Write these on a piece of paper: "GG", "GB", "BG", "BB"
These are the FOUR starting options.
Put them in a bag and pick one.
What are your chances of picking a mixed pair versus GG?

GB and BG are the same, so only count as ONE OPTION

no there arent twice as many mixed

Yes there are. Look. there's two, count them.

girl/boy is the same as boy/girl

No it's not.

if someone asks you how many kids youve got it dosent matter wether you say "a girl and a boy" or "a boy and a girl" there are two ways of saying it but they both mean the same thing

I've got two girls.

You're wrong, but feel free to be shouty and abusive some more. You'll still be wrong.

Go back and look at the tree diagrams again. There are three routes to the 'finish', two of which are one boy and one girl. Two out of three...

no there arent twice as many mixed you CRETIN. girl/boy is the same as boy/girl.

Yes, yes there are. really. Even Smee accepts this FFS.

Okay let's start with 100 parents and no children, okay?
In this puzzle the chances of boy or girl being born are exactly 50/50.

So if those 100 parents all have ONE child then we will have:
50 families with a girl and 50 families with a boy.

Agreed?

Now if the 50 families who have a girl all then have a second child, then they will be split into 25 families with two girls and 25 families with a girl and a boy.

Likewise if the 50 that had boy then have a second child, then they will be split into 25 with two boys and 25 with a boy and a girl.

So the 100 families now look like:
25 with two girls, 50 mixed, 25 with two boys.

400 ???