A fun probability bet.

[marbles]

I remember this one from when I first started learning about probability. You might want to try it out sometime:

You're in a group of 20 people. No one knows anyone else's birthday, but it is widely known that none of them were born on February 29th. Your friend offers to bet you that no two people have the same birthday (month and date only). Hungry for action, he even offers you 3-to-1 odds. Should you take the bet?

[BruceZ]

Of course. I would even take half those odds. I'm only a 1.4-1 dog. I'd need 3-1 odds if there were only 15 people. If there were 23 people, it would be an even money game.

[marbles]

That is correct. In fact, if the population is 23 rather than 20, you're actually a favorite! If the calculator didn't swear by it, I wouldn't believe it myself.

[BruceZ]

I mentioned this one before. Give each player at a 10 person table a full deck of cards. Have them each pick a card. Bet them that two people will draw the exact same card, both rank and suit. You will win 60% of the time.

[marbles]

Hell, why not throw in the jokers and "hoyle's rules of card games" card? You're still over 56%!

Okay, now I'm really starting to feel geeky.