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# Chances of landing 9 tails in a row? watch

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1. Having an argument with a friend about the odds/chances of it happening. Given that it's a 50/50 chance of heads/tails what would the odds of landing 9 tails in a row be? Expressed as a fraction
2. , no?

I always have sucked at probability, though.
3. I have no calculator handy...
4. (Original post by ashy)
, no?

I always have sucked at probability, though.
I know how you feel. I havn't done stats in a month and for a second Poisson flashed through my mind...
5. Yeh it's just (1/2)^9, which is 1/(2^9) which is 1/512.
6. (Original post by impervious)
I have no calculator handy...
Calculator? 1/512
7. Calculator - why? Its 1 in 512. Edit: damn my slowness in typing
8. (Original post by impervious)
I know how you feel. I havn't done stats in a month and for a second Poisson flashed through my mind...
I have a physics degree, and for a brief moment the binomial expansion drifted through my mind. Why? Stupid brain.
9. (Original post by rnd)
Calculator? 1/512
Not so hot at mental maths.. and I wasn't thinking :P You wouldn't believe me if I said I took further maths would you?
10. (Original post by impervious)
Not so hot at mental maths.. and I wasn't thinking :P You wouldn't believe me if I said I took further maths would you?
Course I would.

I'm not much good a mental maths either but I've accidentally memorised quite a lot of useful stuff including powers of 2.
11. The probabillity of getting 9 tails in a row is the same as getting 5 heads and 4 tails.

Coins don't have memory of what side it has landed on.

P.S. Thats not including the probability that it lands on the edge.
12. (Original post by Simplicity)
The probabillity of getting 9 tails in a row is the same as getting 5 heads and 4 tails.

Coins don't have memory of what side it has landed on.

P.S. Thats not including the probability that it lands on the edge.
But do you know the probability of a given sized coin landing on its edge? .
13. (Original post by Simplicity)
The probabillity of getting 9 tails in a row is the same as getting 5 heads and 4 tails.

Coins don't have memory of what side it has landed on.

P.S. Thats not including the probability that it lands on the edge.
The probability of getting 5 heads and 4 tails is:

9C4 multiplied by 0.5^9 = 126/512 = 63/256

Thats because there are 126 different ways in which you can get 5 heads and 4 tails, and only one way in which you can get 9 tails. Of course, the probability of getting 5 heads and 4 tails is the same (1/512)
14. (Original post by little_wizard123)
But do you know the probability of a given sized coin landing on its edge?
I think the main thing is the spin of the coin and not the size. Also, the weight of the coin.
15. (Original post by Simplicity)
The probabillity of getting 9 tails in a row is the same as getting 5 heads and 4 tails.
Actually, the probability of getting 5 heads and 4 tails is 126 times larger than the probability of getting 9 tails.

If you had said "the probability of getting 5 heads followed by 4 tails", you would have a point.
16. Thats what I meant
17. (Original post by Simplicity)
I think the main thing is the spin of the coin and not the size. Also, the weight of the coin.
Sorry, I meant size as "would you know the probability of a 2p coin landing on its side". It was more of a joke really.
18. What I thought =]
Thanks guys.
19. To be fair if a human flips a coin it will biased a bit towards heads. So the probability of flipping a coin and it landing on heads is not 50% more like 51%.
20. (Original post by Simplicity)
To be fair if a human flips a coin it will biased a bit towards heads. So the probability of flipping a coin and it landing on heads is not 50% more like 51%.
Why?

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Updated: July 31, 2009
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