Thread: 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.

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

Code:

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

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?

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.

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...

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

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.

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