Party question

This is a discussion on Party question within the A Brief History of Cprogramming.com forums, part of the Community Boards category; I was on a house warming party this weekend and becaus most people I know are geeks some math questions ...

  1. #1
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    Party question

    I was on a house warming party this weekend and becaus most people I know are geeks some math questions popped up. I usally do pretty well on those but it was this one question I had a hard time to understand why the answear was what it was.

    How many people does it need to be at a party for the chance for 2 people to have the same birthday to be 50%.?

    The correct answear acording to the person asking the question is 22. Somehow I have a mental block on this. Anyone out there know why? I might not ask the question exactly as intended.

  2. #2
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    Well I come from plymouth and the only math our parties involve is:

    Code:
    Satisfaction == Volume of spliff * Percentage Weed || Girls

  3. #3
    5|-|1+|-|34|) ober's Avatar
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    Re: Party question

    Originally posted by Barjor
    How many people does it need to be at a party for the chance for 2 people to have the same birthday to be 50%.?
    Maybe I'm missing something... but that doesn't make a damn bit of sense to me... can you rephrase?

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    Do you know about any houses for sale and well paid jobs in plymouth?

    Lets see if I can make it clearer.

    If I go to a party. How many people does it need to be on that party for the chance to be 50% that two people in that party have the same birthday.

  5. #5
    5|-|1+|-|34|) ober's Avatar
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    hmmm... and the answer was 22??

    sounds like some fishy statistics stuff goin on... I didn't spend a whole lot of time paying attention in that class...

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    Well sqrt(365) is roughly 20 so maybee when the question been told acouple of times the answear changed alittle bit..Not sure

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    I know, I know!!!

    Hey, I think I actually know this one! Who-hoo! ...

    Get to the answer, willya?

    Well, here it is:

    There are really only 2 possibilities here: either someone in the room shares a birthday with somebody else, or everyone has a different birthday. Now, the probability that you will get people that share a birthday is (100% - X) where X is the probablity that everyone has a DIFFERENT birthday.

    ...for some calculations...

    in order to find the probability for a same birthday, take the probability of all different ones, and subtract that from 100%. To find the chance of all different birthdays, it goes lke this:

    1 person > 365 possible days a year
    2 people > 365 for first, 364 for 2nd
    3 people > 365 for first, 364 for 2nd, 363 for 3rd
    ...

    so say you got 10 people... what's the chance someone shares a birthday?

    Answer: 1 - ((365*364*363*362*361*360*35*358*357*356)/(365^10)) = 11.69%

    If any of this makes no sense, i could try to explain this differently. All this here is is probability... yeah.

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    Facinating..I should have known that but nevertheless you helped me from getting more grey hairs. Thanks

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