For the mathematically minded

[AaronBrown]

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Assuming "fair" means "between .495 and .505" or some other suitable approximation, I'll take that bet. 5 in 10,000,000 isn't so unlikely that it's never happened in the history of the world.

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I won't give you the 0.495 to 0.505, what I mean is I would bet that the coin has greater than 0.5 chance of coming up heads.

It's true the world has been around for a while, but how often does someone flip a coin 21 times in a row? It could well be in the millions, or billions, in which case you're probably right that someone hit 21 heads in a row with a coin that had 0.499999 chance of coming up heads. I got a bit carried away. Let me reduce my bet to, if I saw a coin come up heads 21 times in a row, I would bet that it had greater than 0.5 chance of coming up heads.