The boy-girl puzzle

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  • The boy-girl puzzle
  • Premier Icon GrahamS
    Subscriber

    As an alternative to the old plane-conveyor one…

    You ask a woman how many children she has and she says two.
    Then for some odd reason you ask her “Is at least one a girl?”, to which she replies “Yes”.

    So what are the odds that she has a girl and a boy?

    (to avoid some argument: assume this is a perfect world where the odds of boy or girl conception are 50/50 and that there are no genetic “third sex” possibilities)

    Premier Icon funkynick
    Subscriber

    66%

    Next…

    jarl
    Member

    75/25. Not sure how on earth you got 66% ๐Ÿ˜›

    Premier Icon funkynick
    Subscriber

    Explanations are just so passe don’t ya know…

    Premier Icon GrahamS
    Subscriber

    And for a bonus point, what would the odds be if you had instead asked “Tell me sex of one of them” and she had said “A girl”?

    (probably best to stick with percentage odds for those of us that can’t do fractions)

    Premier Icon funkynick
    Subscriber

    50%

    And next…

    Premier Icon Bunnyhop
    Subscriber

    Why aren’t you asleep and tucked up in bed?

    Premier Icon funkynick
    Subscriber

    One could ask the same question… ๐Ÿ˜‰

    But I am in bed at least, just wish I could sleep…

    Premier Icon mboy
    Subscriber

    50%, 1/2, half, whatever you want to call it.

    People, for once and all, something that has already happened (in this case the sex of one child has been announced) CAN NOT EVER influence the probability of something happening in the future.

    As we know one child is a girl, the options in this case are thus:

    Girl + Girl
    Girl + Boy

    As the first child being a girl DOES NOT influence the probability of the 2nd child’s sex, and the quoted probability of any further child being 50/50, then this is the probability of there being a boy and a girl.

    funkynick, how the hell did you get 66%? Did you pay attention to maths lessons in school?

    jarl, same comment as for funkynick.

    God why don’t people understand probability! ARGH!!! I have presented to some very high level people in big corporations, on large salaries several times talking about probability, and almost without exception they NEVER seem to understand that the fact that an event has happened (ie. flipping a coin, the sex of a child etc etc.) DOES NOT influence any upcoming events!

    You flip a coin 4 times in a row, you get 4 straight heads. What’s the probability of getting a head on the 5th attempt?

    Answer is it’s 50%, unless you have a 2 headed coin, in which case it would be 100% ๐Ÿ˜‰

    Though don’t confuse this with working out the probability of flipping 5 straight heads (on a coin with a heads and a tails on it) before any flipping occurs. This would of course be 1/2 x 1/2 x 1/2 x 1/2 x 1/2 = 1/32 (or 0.03125 for those that only do decimals).

    Oh, and as for the plane on a conveyor, the wheels on a plane have nothing to do with its ability to take off or not. Air speed over the wings is what matters!

    Aidy
    Member

    mboy, I suspect you need to review your grasp of probability.

    The subtley lies in that the children are not the same child.
    By the question “is at least one of them a girl”, you allow for the chance that this could refer to either of them. We aren’t dealing with probability of future events, as in your coin toss example, but of a predefined existing situation.

    If we only knew that she had 2 children, we’d have the following possibilities:

    Boy/Girl
    Boy/Boy
    Girl/Boy
    Girl/Girl

    As we know that at least one is a girl, it rules out the Boy/Boy scenario.
    Of the scenarios we have left, two involve one boy, and one girl. Resulting in a chance of 2 thirds.

    Premier Icon mboy
    Subscriber

    Aidy

    That’ll learn me to read the subtleties of the question! ๐Ÿ˜ณ

    Yes sorry, pre-defined existing situation is indeed totally different.

    Apologies to funkynick too in that case.

    Maths/probability is/was my strong point, my understanding of verbal reasoning has never been quite so strong.

    Well worded question GrahamS! Caught me napping at least.

    bigrich
    Member

    sex of the offspring is highly influenced by enviromental factors. where does she live?

    Premier Icon funkynick
    Subscriber

    No worries mboy… now I wonder if there is a smug smiley? ๐Ÿ˜‰

    Conor
    Member

    Bunnyhop – Member

    Why aren’t you asleep and tucked up in bed?

    How’s the snow?

    Premier Icon MikeG
    Subscriber

    If we only knew that she had 2 children, we’d have the following possibilities:

    Boy/Girl
    Boy/Boy
    Girl/Boy
    Girl/Girl

    As we know that at least one is a girl, it rules out the Boy/Boy scenario.
    Of the scenarios we have left, two involve one boy, and one girl. Resulting in a chance of 2 thirds.

    Surely in this situation Boy/Girl is the same as Girl/Boy? The question doesn’t place any importance on the order of the children so if ‘at least one is a girl’ the only possibility for the other child is to be either a boy or a girl. We know this is a 50/50 chance.

    Although I last did maths in anger over 10 years ago and I’m quite hungover this morning so I might be spouting rubbish.

    Premier Icon Drac
    Subscriber

    I can’t see why the order matters either so there only 2 possible combinations.

    coffeeking
    Member

    But then isnt the probability of “a boy/girl combo, in any order” higher than boy boy or girl girl, twice as high, so the maths still works out?

    Premier Icon matthewjb
    Subscriber

    I can’t see why the order matters either so there only 2 possible combinations.

    The order does matter. The youngest being a boy and the oldest being a girl is a different scenario from the youngest being a girl and the oldest being a boy.

    So that gets us back to Aidy’s 4 possibilities:

    Youngest/Oldest

    Boy/Girl
    Boy/Boy
    Girl/Boy
    Girl/Girl

    Boy/Boy is excluded so the probability is 2/3rds

    Premier Icon GrahamS
    Subscriber

    And so it begins….

    Smee
    Member

    50:50 end of. Any other solution is just stupid.

    Is one one a girl? Yes. Is the other one a boy? 50:50 chance.

    Premier Icon miketually
    Subscriber

    Oh, and as for the plane on a conveyor, the wheels on a plane have nothing to do with its ability to take off or not. Air speed over the wings is what matters!

    You’re correct on both of those. But that still doesn’t answer whether or not it takes off. I want to know what, because I’d love it to turn out that you’re useless with physics as well as verbal reasoning after your rant about people not understanding probability above ๐Ÿ™‚

    nuggett96
    Member

    The original question said nothing about order that they came in just that she has two and one of them is a girl.

    so…
    girl/boy
    boy/girl are the same in this instance, surly it’s 50/50

    Premier Icon miketually
    Subscriber

    Is one one a girl? Yes. Is the other one a boy? 50:50 chance

    Maybe you should read the question again?

    Smee
    Member

    mike – tell me what i missed.

    Premier Icon Drac
    Subscriber

    The order does matter. The youngest being a boy and the oldest being a girl is a different scenario from the youngest being a girl and the oldest being a boy.

    No one asked about the age.

    coffeeking
    Member

    mike – tell me what i missed.

    The question asks if “AT LEAST one is a girl”.

    That means you can have 3 out of the 4 possible combinations. The question didnt ask:

    Which one comes first?
    If the first is an X, whats the second?
    How many cows does it take to provide enough milk for the family?

    etc

    Premier Icon GrahamS
    Subscriber

    Oh, and as for the plane on a conveyor…


    DON’T CROSS THE STREAMS!

    coffeeking
    Member

    And for completeness, planes only take off due to airspeed over the wings, so the plane still has to reach the same take-off speed and the conveyor makes no difference (assuming no friction in the bearings/wheels) to what the engine has to do.

    Smee
    Member

    Who gives a shit which comes first? The question doesn’t ask that.

    Premier Icon funkynick
    Subscriber

    Smee… I’m sure the children do! ๐Ÿ˜€

    Premier Icon Sandwich
    Subscriber

    Dad usually, though if he’s a gentleman then it could be mum ๐Ÿ˜ฎ

    Premier Icon GrahamS
    Subscriber

    Yep funkynick knows his stuff.

    The correct answer is of course 66% (well 66.6666..% actually but you get the point)
    And in the second bonus question the answer is 50%.

    Anything else is wrong wrong wrong.

    Premier Icon funkynick
    Subscriber

    Woooot.. do I get a prize? ๐Ÿ˜€

    coffeeking
    Member

    My head hurts.

    You know she has 2 kids.
    She states at least one is a girl.

    So the other must be either boy or girl (50% chance). We ask whats the odds she has a girl and a boy, the answer is 50% because we already know one (at least) is a girl and we know the probability of a single event and only want one of the two possible outcomes.

    I used to like probability problems, but this has me confused.

    As for the bonus point, I dont see it makes any difference, youve still effectively ascertained that one is a girl and the other could be either?

    More detailed explanation needed

    Premier Icon Drac
    Subscriber

    I’m still lost why it’s 66% as the order was never discussed it only if you specify the order will the odds change surely?

    spoon
    Member

    I’m sure what we are after here is some baysian analysis..

    Premier Icon GrahamS
    Subscriber

    Related point:

    Given a group of parents with two children then 75% of them will have at least one girl. But in that same group, 75% will also have at least one boy!

    Premier Icon Drac
    Subscriber

    Discused here the ambiguity also seems to be with how the question is asked.

    coffeeking
    Member

    I’m bored now, explanation of the original point or **** off ๐Ÿ™‚

Viewing 40 posts - 1 through 40 (of 463 total)

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