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Coin-flipping game (self.dailymaths)

submitted 7 months ago* (last edited 7 months ago) by zkfcfbzr to c/dailymaths

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We're playing a game. I flip a coin. If it lands on Tails, I flip it again. If it lands on Heads, the game ends.

You win if the game ends on an even turn, and lose otherwise.

Define the following events:

A: You win the game

B: The game goes on for at least 4 turns

C: The game goes on for at least 5 turns

What are P(A), P(B), and P(C)? Are A and B independent? How about A and C?

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[–] [email protected] 3 points 7 months ago (1 children)

im not sure i understand either. could you tell me what dependence and independence mean in this context? perhaps that is the source of my confusion. i understood it in a naive sense of one variable depending on the other for its outcome, and that whether the evens player wins is simply dependent on the number of the round. B therefore A and C therefore not A seem like tautological and equivalent dependencies(?) if i understand the question, and there is a good chance i do not. Ah, but i reread "at least" now, which changes everything.

Yes, i'm quite certain now: I do not understand.

also, how come i got the right probability for the players' winrate if my calculation of that was based on one over two to the n instead of the n minus one? Why didnt the off by one error carry forward giving the odds player the 1/3 chance and vice versa?

like i said, dont know nuffin.

[–] zkfcfbzr 4 points 7 months ago* (last edited 7 months ago)

In probability, two events are said to be independent if one event happening has no effect on the probability of another event happening. So coin flips, as an example, are independent - because when you flip a coin and get Tails, that doesn't affect the probability of the next coin flip also coming up Tails.

So in this context, asking if A and B are independent is asking: Does knowing the game lasts at least four turns change the probability of winning? And similarly for A and C. Does knowing the game lasts at least 5 turns change the probability that the game will end in a victory?

Rest of response

To be clear about your other answers, saying P(B) = 1/16 and P(C) = 1/32 are not correct - I was saying if you adjusted your formula, from 1/2^n to 1/2^(n-1), then your answers would be correct. So the probability of the game lasting at least four turns is 1/2^(4-1) = 1/2^3 = 1/8, and the probability of it lasting at least 5 turns is 1/2^(5-1) = 1/2^4 = 1/16.

But I think you were more asking about why this didn't affect your win rate - that's because there's a subtle difference between "making it to the nth round" and "having the game end on the nth round" - and that difference is that once you make it to a round, you then have a 1/2 probability to end the game - which makes the probability of ending on the nth round the 1/2^n you used.