# HowStuffWorks "Someone told me that if there are 20 people...

Someone told me that if there are 20 people in a room, there's a 50/50 chance...

842K views howstuffworks.com## Interest

on Sep 9, 2007

# Stumblers Who Commented On This Page

## ungysutherland

### ungysutherland

27.027763503479158569444627848114

## Choco-Dip

### Dip

That's so cool!!!!!

## Jasmine124

### Jasmine124

I like this in this page- This phenomenon actually has a name -- it is called the birthday paradox, and it turns out it is useful in several different areas (for example, cryptography and hashing algorithms). You can try it yourself -- the next time you are at a gathering of 20 or 30 people, ask everyone for their birth date. It is likely that two people in the group will have the same birthday. It always surprises people!

## ShantyShanty

### Chantelle

Interesting post

## palvarez

### Leo

This is not right. For this to work, if you are one of that room you should ask to 19 other people not in that room, like each other should do. Because you took 1 answer (and 1 posibility of each other). So, the next person, when ask to you, he will got the same answer as you got when you ask him. So, the next one will have 18 posibilities of matching dates. And the next one 17. The probabilities cancel with each other.

## gerakis100

### Udi

Statistics can be fun!

## LuigiCircuit

### LuigiCircuit

Not a paradox, but still cool.

## schmudark

### schmudark

What about certain months not being as popular for baby making

## StumblerAustin

### Austin

well.. out of all the classes ive been in where we arranged ourselves by birthdays, only one person's ever shared my birthday, and a select few my birth MONTH. must be due to the fact that my month isnt very popular for births.

## KingOfSporkdom

### KingOfSporkdom

By no means a paradox, and I'd known about this for a while (though the smallest number for which a common birthday becomes probable is 23), but I hadn't known how to figure the probability. This is a good explanation for that, if nothing else.

## Vincexstoops

### Vincent G

So we get married next?

## zephranthes

### Craig

Well that was severe over-thinking.

## sherrysplace

### Sherry

Very interesting!

## mchalter

## mchalter

I tried it twice.. It didn't work