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Birthdays of grandfather and granddaughter

  1. Feb 17, 2004 #1
    Hi to all you smart people out there: I need your help, and don't know how to solve this. If a grandfather is born on February 4, 1950 at ll:07 a.m. and his granddaughter is born on February 4, 2002 at ll:07 a.m. how do I find how the probability or stats of this happening between birthdays of grandfather and granddaughter. I really appreciate your help. Thank you very much.
     
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  3. Feb 17, 2004 #2

    matt grime

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    do you mean the same day, hour, minute or what. the question as posed is too vague and has many possible interpretations. the simplest is, given your grandfather's DoB, then there is a chance of 1 in 365 that your granddaughter will be born on the same day in the year
     
  4. Feb 17, 2004 #3

    NateTG

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    It also depends on what kind of assumptions you make about average life spans, average age of parents at the time of giving birth, whether you include all time zones, and various other things.

    If you have a notion of how precisely the two are aligned, and you have a probability function, then you can start using numbers.
     
  5. Feb 17, 2004 #4
    Thanks Matt, I guess I am asking what are the chances that a grandaughter would be born on the same birthdate, and time as the grandfather? I don't know how to setup the problem. He is born 2/4 and she is born on 2/4 both at ll:07 a.m. I am not very good at math or expressing myself. Thank you.
     
  6. Feb 17, 2004 #5

    NateTG

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    freclesan - If you assume that the probability distribution is flat (which is not necessarily valid), then the answer is:
    1/365 for the right day (since 2002 is not a leap year)
    1/(365*24) for the right hour
    1/(365*24*60) for the right minute
    1/(365*24*60*60) for the right second
    and so on.
     
  7. Feb 17, 2004 #6
    Hi, so then if I multiply 1/(365*24*60) = 525600 (I don't think they know the seconds) then it means there is a l in 525600 of his being born on the same month, day and time (hour plus minutes) as his granddaugher, right? I really appreciate this very much. You people have made my day!
     
  8. Feb 18, 2004 #7

    selfAdjoint

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    But what you have to do is work out the probability they are not born in the same time period and then take 1 minus that. It's just the old what's the probability there are two people in a class with the same birthday. For a class of 25 the probability is 50%.
     
  9. Feb 18, 2004 #8

    NateTG

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    Actually, the probability for a random group of 22 people to have a pair with the same birthday is already better than 50%.

    Since there are only two people in this problem - the daughter and the grandfather, it's a bit simpler.
     
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