View Full Version : Party question

03-18-2002, 12:37 PM
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.

03-18-2002, 12:40 PM
Well I come from plymouth and the only math our parties involve is:

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

03-18-2002, 12:52 PM
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?

03-18-2002, 12:55 PM
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.

03-18-2002, 12:59 PM
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...

03-18-2002, 01:01 PM
Well sqrt(365) is roughly 20 so maybee when the question been told acouple of times the answear changed alittle bit..Not sure

03-18-2002, 02:51 PM
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.

03-18-2002, 03:39 PM
Facinating..I should have known that but nevertheless you helped me from getting more grey hairs. Thanks