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comment by Devac
Devac  ·  917 days ago  ·  link  ·    ·  parent  ·  post: The fake coin probability problem

0.5 * (1 + 0.5 ^ k),

so k + 1-th is just

0.5 * (1 + 0.5 ^ (k + 1))

So for k = 3 it's 0.5 * (1 + 1/8) = 9/16

For k = 4 it's 0.5 * (1 + 1/16) = 17/32

As you can see, it steadily goes down, for very high number of flips it will get very close to 0.5, but still marginally above 0.5. The limit for this formula in sense of k -> Infinity is of course 0.5, but you can't flip infinitely many times.

EDIT: I think that I have messed up, but I can't put my finger on it. If it's any excuse, I was never any good with probability. How does one differentiate consecutive case from consecutive and a next flip? This smells of Markov Chain to me, but that's just a hunch.

mike  ·  917 days ago  ·  link  ·

Something to think about: each flip of the coin where you see heads changes the probability you've got the fake coin. If you flipped the coin 100 times and got heads every time, I think you'd be about 100% certain that the next time it will be heads.

snoodog  ·  917 days ago  ·  link  ·

I thought of it the same way Devac thought about it but given the hint it comes down to something like this.

Edits for formatting

Turn Fake ------- Real Probability

1 1/2 ---- 1 / (.5 +1)

1 1/4 ------- 1/(.25+1 )

1 1/8 --------- 1/(.125+1 )

1 1/16 -------- 1/(.0625+1 ) -94%

1 1/2^N ---------- 1/(.5^N+1 )