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Posers and Puzzles

Posers and Puzzles

  1. Standard member wolfgang59
    Infidel
    03 Mar '14 21:54
    I posted a similar problem on here a couple of years ago, if you
    remember that don't contribute, just sit back and enjoy the discussions!

    I phone a work colleague up and his daughter answers the phone. He has
    previously told me that he has two children at home, I had no idea he had
    a daughter. What are the chances that the other child is also a girl?
  2. Subscriber joe shmo
    Strange Egg
    03 Mar '14 23:58
    Originally posted by wolfgang59
    I posted a similar problem on here a couple of years ago, if you
    remember that don't contribute, just sit back and enjoy the discussions!

    I phone a work colleague up and his daughter answers the phone. He has
    previously told me that he has two children at home, I had no idea he had
    a daughter. What are the chances that the other child is also a girl?
    I'm going to stick my neck out here and say the probability is 1/3?
  3. Standard member talzamir
    Art, not a Toil
    04 Mar '14 14:20
    Agreed =)
  4. Subscriber Ponderable
    chemist
    04 Mar '14 14:29
    Well the items are (elder sibling/younger sibling):

    b/b (evidently untrue)
    g/b
    b/g
    g/g

    so 1 in three should be correct.
  5. Standard member forkedknight
    Defend the Universe
    04 Mar '14 16:00 / 2 edits
    b/b (b1 answers) nope
    b/b (b2 answers) nope
    b/g (b answers) nope
    b/g (g answers)
    g/b (g answers)
    g/b (b answers) nope
    g/g (g1 answers)
    g/g (g2 answers)

    I'm going to say 1/2

    Why would it be any different than:
    "I flip a coin, and it comes up heads, what are the chances that my next flip is heads?"

    You could use the same logic that you guys just used if you had already flipped the coins and were asking after the fact.
  6. Standard member forkedknight
    Defend the Universe
    04 Mar '14 16:06
    Note that this is different than the problem:

    I have four socks, two black and two blue. I randomly put to of the socks in a bag.

    You grab one sock out of the bag, and it is black. What is the probability that the other sock is black?
  7. Subscriber joe shmo
    Strange Egg
    04 Mar '14 22:19
    So... before the phone call the probability that both of the children were girls was 1/4.

    After the phone call the probability that both of the children are girls is 1/3 and the probability the the other child is a girl is 1/2.

    Is this a correct line of reasoning, or not?
  8. Standard member forkedknight
    Defend the Universe
    05 Mar '14 00:00 / 1 edit
    Originally posted by joe shmo
    So... before the phone call the probability that both of the children were girls was 1/4.

    After the phone call the probability that both of the children are girls is 1/3 and the probability the the other child is a girl is 1/2.

    Is this a correct line of reasoning, or not?
    You know the first child is a girl.

    If the other child is a girl, then they're both girls. How could the probabilities be different?
  9. Subscriber deriver69
    Keeps
    05 Mar '14 08:17
    I know that if this person has two children, one of the a girl called sue then the probability the other one is a girl is 1/3.

    If a person has two children, the oldest one is a girl called sue then the probability is now 1/2.

    I would go with the BB, BG, GB, GG explanation and go with 1/3 however often the oldest child is the one who answers the phone.
  10. Standard member forkedknight
    Defend the Universe
    05 Mar '14 15:25
    Let's take an infinite set of families with two children, where children have an equal probability of being boys or girls, and you ask the question:

    "Of all of the families with at least one girl, what is the probability that both children in that family are girls?"

    Then the answer is 1/3

    If, on the other hand, you randomly choose one family, and then randomly sample one of the children, and discover that child is a girl, then the probability that the other child is a girl is not conditionally related to your sample and the probability is 1/2.
  11. Standard member wolfgang59
    Infidel
    05 Mar '14 19:46
    KEEP GOING GUYS
  12. Subscriber deriver69
    Keeps
    06 Mar '14 02:22
    Of course if you phone the work colleague up somewhere that is not his home it could be one of his 5 children that have left home.

    If you have four two children families BB, BG, GB, GG. You take a random sample of one by for instance phone. There is only one case in which the other child matches your sample (and one case which is clearly excluded by the result of the sample). So I am still sticking with 1/3.
  13. Standard member wolfgang59
    Infidel
    06 Mar '14 02:39
    Originally posted by deriver69
    Of course if you phone the work colleague up somewhere that is not his home it could be one of his 5 children that have left home.

    If you have four two children families BB, BG, GB, GG. You take a random sample of one by for instance phone. There is only one case in which the other child matches your sample (and one case which is clearly excluded by the result of the sample). So I am still sticking with 1/3.
    1/3 is incorrect. Think again.

    In fact it may help to consider the same problem ... but with gender reversed!
  14. Standard member talzamir
    Art, not a Toil
    06 Mar '14 12:17
    I thought again.. yep. 50%
  15. Standard member smw6869
    Granny
    06 Mar '14 21:07
    50/52


    GRANNY.