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The Tuesday Birthday Problem 981

An anonymous reader sends in a mathematical puzzle introduced at the recent Gathering 4 Gardner, a convention of mathematicians, magicians, and puzzle enthusiasts held biannually in Atlanta. The Tuesday Birthday Problem is simply stated, but tends to mislead both intuitive and mathematically informed guesses. "I have two children, one of whom is a boy born on a Tuesday. What's the probability that my other child is a boy?" The submitter adds, "Believe it or not, the Tuesday thing is relevant. Well, sort of. It's ambiguous."
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The Tuesday Birthday Problem

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  • Re:Rubbish (Score:3, Interesting)

    by Securityemo ( 1407943 ) on Tuesday June 29, 2010 @05:38AM (#32727928) Journal
    Well, if it isn't the same as saying that, then it's a distortion of information expression or an error in method, in the sense that the mathematics don't fit reality when it's your intention that they do so?
  • by jimicus ( 737525 ) on Tuesday June 29, 2010 @06:54AM (#32728342)

    It's playing games with words and attaching significance to two sets that in any practical case I can think of would be considered one.

    The argument is that if you were to consider it as a set, there are four possible ways for your children to be distributed:

    1. (Boy, Boy)
    2. (Boy, Girl)
    3. (Girl, Boy)
    4. (Girl, Girl)

    We already know that your children can't possibly fall into the fourth set, and so looking at the sets it appears that the probability should be 1/3. But this misses one minor point - you've added an extra set which only makes sense if you wish to attach significance to the order in which the children were born (Sets 2 and 3). But as soon as you do attach that significance, the information you are given in order to establish the probability of any particular outcome (eg. the boy is older) allows you to eliminate two sets rather than just one.

  • Re:Probability (Score:3, Interesting)

    by dargaud ( 518470 ) <> on Tuesday June 29, 2010 @07:30AM (#32728590) Homepage

    There is a very small chance it will land balanced perfectly on it's side

    Has anyone ever seen that happen?

    Yes, I've had that happen once with a 10Fr coin (very similar in shape to the current 1 euro coin). The ground was irregular which probably helped a lot.

  • Re:Well? (Score:2, Interesting)

    by repapetilto ( 1219852 ) on Tuesday June 29, 2010 @07:30AM (#32728592)

    Indeed, is everyone drunk or something...

    First, The question doesn't say the other (this does not mean older or younger...) child was not born on a Tuesday, maybe the questioner meant to include this info but they failed to.

    Second, the probability that the other child is a boy is either 1 or 0, it's something that has already occurred... The questioner probably meant to ask "What is the probability that if you guess the other one is a boy you will be correct?"

    So if we correct the question to read as it was likely intended to be read:
    "I have two children, one of whom is a boy born on a Tuesday. The other child is not a boy born on a Tuesday. What is the probability that you will be correct if you guess that my other child is a boy?"

    So you think to yourself, well assuming boys pop out of that mom just as easily of girls and doesnt prefer to do it any particular day of the week, that means its 1/7 chance it might have happened on any given day and the likelihood it was a boy is 50:50 for 6 days of the week and then 0 for tuesday. So you multiply .5 by 6, add zero, and take the average (divide by 7) to get 3/7.

    The real confusion occurs due to the use of odd numbers... Imagine a world where everything was found in sets of twos, people had 2 heads, 4 arms, etc. They would always be dealing with eating animals that were siamese, if they wanted to hunt by throwing rocks or whatever each siamese would throw a rock so they would use two rocks. In this world I would say that what we call the number 2 would actually be like their number 1, and what we use as unity, or one, would be for the siamese called a half. Therefore their numberline would go 0, .5, 2, 2.5, 4, 4.5, 6, 6.5, 8, etc.

    This is actually more reflective of reality in that, deep down, math and counting are extensions of logic, and the fundamental unit of logic is a true-false statement which is basically a set of 2. True is only 1/2 of the total possibilities for any given logical statement. For example say you have counted one rock, what that actually represents is both having one rock in your presence butt also, concurrently, not having counted other than one rock, so in essence you have counted two different things and are representing them with a number supposed to correspond with one thing. Wouldnt it make more sense to just use "two" to represent the one thing youve counted?

    The probability of guessing correctly by saying the second child is a boy would therefore be 1/2(6), or 3, divided by 6 and a half, which gives you 6 out of 12 and 1/2 odds.

  • by Anonymous Coward on Tuesday June 29, 2010 @11:16AM (#32731474)

    The comment you quoted is incorrect. There are 28 combinations of boy/girl and day of the week, but only 27 unique combinations. Here they are:

    1 Boy Thu, Girl Sun
    2 Boy Thu, Girl Mon
    3 Boy Thu, Girl Tue
    4 Boy Thu, Girl Wed
    5 Boy Thu, Girl Thu
    6 Boy Thu, Girl Fri
    7 Boy Thu, Girl Sat
    8 Boy Thu, Boy Sun
    9 Boy Thu, Boy Mon
    10 Boy Thu, Boy Tue
    11 Boy Thu, Boy Wed
    12* Boy Thu, Boy Thu
    13 Boy Thu, Boy Fri
    14 Boy Thu, Boy Sat
    15 Girl Sun, Boy Thu
    16 Girl Mon, Boy Thu
    17 Girl Tue, Boy Thu
    18 Girl Wed, Boy Thu
    19 Girl Thu, Boy Thu
    20 Girl Fri, Boy Thu
    21 Girl Sat, Boy Thu
    22 Boy Sun, Boy Thu
    23 Boy Mon, Boy Thu
    24 Boy Tue, Boy Thu
    25 Boy Wed, Boy Thu
    26* Boy Thu, Boy Thu
    27 Boy Fri, Boy Thu
    28 Boy Sat, Boy Thu

    Note that Boy-Thursday-Boy-Thursday occurs twice (12 and 26). The article (and the quoted comment) incorrectly ignores the second instance because it is not unique, leaving only 27 combinations. The assumption they make is that Boy-Thursday-Boy-Thursday is equally as likely as the other 26 options, which is not true. In fact, the Boy-Thursday-Boy-Thursday is twice as likely because you do not know which son was introduced.

    If the second Boy-Thursday-Boy-Thursday is included, the probably that the other child is a boy becomes 14/28, or 50%.

BLISS is ignorance.